Rail Transportation Systems: Heavy-haul rail transport





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1. DEFINITION AND GENERAL DESCRIPTION OF THE SYSTEM

The term 'heavy haul rail transport' describes any railway operation using trains of a mini mum axle load equal to 25t (Q = 25-40t).

Trains that are intended for carrying heavy loads operate either in dedicated freight rail way corridors or in corridors with mixed traffic operation. They serve the transport of conventional and hazardous goods. Railway networks intended for heavy haul transport satisfy all categories of cargo movement and usually have a broad or normal track gauge ( FIG. 1). The heavy haul rail transport requires reduced running speeds (Vmax = 80-100 km/h).

Table 1 presents, for various heavy haul railway corridors, the number of tracks and the track gauge used, the length of the connection they serve, the allowed axle load, the total weight of the train, the traffic volume that they transport annually, the features that are selected for the track superstructure and the maximum running speed of freight trains.

In Section 3 (below), constructional and functional characteristics of heavy haul rail transport systems that differ from those of conventional freight rail systems are identified. The heavy haul transport differs from other rail freight activities and should be considered and treated separately from the transportation of conventional loads.

2. THE INTERNATIONAL MARKET IN HEAVY HAUL RAIL TRANSPORT

Heavy trains are commonly used in America, South Africa and Australia, where heavy haul rail transports have the largest share in the rail freight market. The train that carries the heaviest loads worldwide is also the longest one and is found in Australia. This train's pay load is 82,000t, its gross weight is 99,734 t and it is composed of 682 wagons and 8 diesel locomotives of 6,000 HP. The train's length is 7.2 km and it transports bulk minerals from one part of Australia to the other across thousands of kilometers, passing through uninhabited areas and deserts .

In the EU, the average hauled weight of freight trains reaches 1,000 t, while in America trains that are 10-20 times heavier are used, the axle weight of which is double than the respective axle weight of trains at EU countries; trains operated in America also are 2-3 times longer than EU ones. In the EU, trains with large axle load have been used for experimental route tests, but in practice such trains have only been launched in Sweden. Their weight approaches 8,160 t (gross weight), and the line where they are used has a length of 43 km; however, this length represents only 0.3% of the total length of the EU railway net works.

The tracks that are used for the heavy haul rail transport either belong to mixed traffic operation networks (India, Russia, etc.), in which the tracks are designed for a high design axle load, or are networks where freight trains have priority which are specially constructed for heavy loads (United States, Canada, etc.).

Countries in which such transportation is widely used are usually countries where the largest volume of transported goods are bulk products, which are related either to the needs of power production, for example, carbon, or to their manufacture, for example, iron ore.

In the EU on the contrary, the transported products are mainly construction materials and chemicals, and for such products there is high competition between the railway and maritime and road transport.

3. DIFFERENCES BETWEEN CONVENTIONAL AND HEAVY HAUL FREIGHT RAILWAY NETWORKS

The operation of heavy haul freight trains on the one hand seems to achieve 'economies of scale' as these trains have a much higher transport capacity than the conventional freight trains. On the other hand, however, such an activity increases the implementation and maintenance cost of the railway track. The reason for this is that many features of the heavy haul freight trains/wagons differ substantially from the respective features of the conventional haul freight trains/wagons.

In columns (1)-(3) of Table 2, the features that differentiate between the two systems are presented.

4. IMPACTS OF HEAVY HAUL RAIL OPERATIONS AND MAIN DESIGN PRINCIPLES

Column (4) of Table 2 lists the impacts, both positive and negative, which cause differentiations between the two systems, as well as the requirements imposed on their design, construction and operation.

The axle load is directly or indirectly involved in the analytical expressions of all forces acting on the wheel-rail contact surface and influences the behavior of both the rolling stock and the track.

The presence of high axle loads creates many technical problems. The transporting of heavy loads results in increased stresses applied on the rails and transferred by the features of the track superstructure to the subgrade. This has serious implications on the rails (such as cracks, damage to the weld points and breakage due to fatigue) and on the sleepers (e.g., wear). Moreover, damage is observed at the areas of switches and crossings. Stresses on the track bed layers and the formation layer are likely to exceed the permissible values.

Therefore, the maintenance cost and the frequency of track inspections are increased.

In order for the heavy haul rail transport to become financially efficient, it is required: (i) for heavy loads' lines that are already in operation to be frequently checked concerning defects of the track superstructure and (ii) for lines that are under construction, all features of the infrastructure to be specially designed. More specifically, as discussed below, the main requirements are the use of heavier rails, the use of sleepers that are of greater mechanical strength, the increased density of the sleeper layout, the special design of the ballast regarding its thickness and the type of materials used, the provision of specifically designed formation layer and the improvement of the soil if it is of poor quality.

For heavy haul rail transport rail (Q > 30 t) the specifications and technical solutions that are described hereunder should be adopted.


FIG. 1 Heavy haul rail transport, Convoy formation.

[coming soon] Table 1 Main characteristics of railway lines intended for heavy haul rail transport (indicatively)

[coming soon] Table 2 Features of the heavy haul freight wagons/trains that differ significantly from those of the conventional haul freight wagon/trains - effects (positive or negative) and requirements for the design, construction, operation and maintenance of the railway system

4.1 Selection of track infrastructure components

4.1.1 Selection of the track's alignment geometric characteristics

In case of broad gauge track, the same track infrastructure can serve much heavier loads than in the cases of normal and metric gauge track.

In practice, an increase in the axle load (up to a certain degree) is not treated by widening the gauge but by increasing the mechanical strength of the features of the track's superstructure (e.g., heavier rails).

The increase of the axle load leads to the adoption of lower longitudinal gradients. The adoption of larger longitudinal gradients in many cases reduces the tunneling works and the construction of bridges considerably; however, it requires locomotives of greater traction power.

4.1.2 Selection of rails

When the axle load is increased, an increase of the cross section of the rail is required, that is rails that are heavier and of greater mechanical strength are required (UIC 60, UIC 72 grade steel 90 kp/mm2).

Based on the mathematical equation (eq.1), it is deduced that axle loads of 20, 25 and 30 t require rails of minimum weight equal to 50, 60 and 70 kg/m, respectively.

An increase of the axle load by 25% requires a 20% increase of the rail weight:

Br = 2.25Q + 3 (16.1) where

Br: rail weight per meter (kg)

Q: axle load (t)

For axle load up to 25 t rails of 60 kg/m 90 UTS (ultimate tensile strength) is sufficient.

For higher axle loads, heavier rails whose cross sections are heavier are required.

For a load Q = 30 t, rails of 68.5 or 71 kg/m must be used.

Regarding the stress that is developed on the rail (and therefore regarding the rail's wear)

it increases as the quantity (Qo)? increases, where ? is exponent, the values of which are between 3 and 4, and Qo is the wheel load.

Longer wavelength irregularities, usually known as 'waves', which appear on the rail's rolling surface are due to fatigue of the wheel-rail contact surface. During the heavy haul rail transport, the pressure on the contact surface is very high, and this results in the development of corrugations troughs (with gross plastic flow), which have a wavelength of 200-300 mm and a frequency of 30 Hz, for average traffic speed. The longer wavelength corrugations have a particularly big impact on the maintenance, as they increase costs by up to 30%.

In any case, it is evident that the increase of axle loads intensifies the phenomena of fatigue and their consequences.

Increased axle load results in wear and fatigue during the early stages of the line's operation.

Regarding the frequency of the required track maintenance work, the influence of vertical loads is catalytic. The deterioration of the track's quality is proportional to the third power of the value of the axle loads. An increase in the axle load by 10% reduces the intervals between two consecutive maintenance works by 30%.

Finally, with regard to the quality of steel, it is required that the steel used be harder and heat treated, in order to be able to bear the increased loads, to have increased resistance against wear and fatigue and to ensure increased lifetime for the rails.

4.1.3 Selection of the type of sleepers and the distances between them

The tracks on which high axle loads are expected to be imposed are usually constructed using sleepers made of prestressed concrete. The sleepers' density is 1,540 sleepers per km or 1,660 sleepers per km. The use of 60 kg/m rails and prestressed concrete sleepers placed at distances of 43 cm between them is suitable for the operation of axle loads that are equal to 30 t.

4.1.4 Selection and dimensioning of track bed layer features

By applying the mathematical equation (eq.2) the impact of the axle load on the thickness of the track ballast and the sub-ballast layers can be assessed as:

ebt = eb + esb = Eb + ab + bb + cb + db + fb (eq.2) where

eb: ballast layer thickness (m)

esb: sub-ballast layer thickness (m) ebt

: total thickness of ballast and sub-ballast layers (m)

Eb: parameter that depends on the quality category of the soil and the bearing capacity of the substructure ab: parameter that depends on the classification of the track in the UIC classes bb: parameter that depends on the sleepers' length and material cb: parameter that depends on the volume of the required track maintenance work db: parameter that depends on the maximum axle load Q fb: parameter that depends on the track design speed Vd and the bearing capacity of the substructure

By using the mathematical equation (eq.2), the contribution rate of the parameter db on the total thickness ebt , and, thus, the influence rate of the axle load Q on it can be derived graphically ( FIG. 2). As shown in FIG. 2 the influence rate of the axle load on the total thickness of ballast and sub-ballast layers is negligible for passenger trains (Q = 20t), while for freight trains it can increase the thickness by 10%-21%.

The thickness of the sub-ballast can range between 15 and 75 cm, depending on the quality of the soil material of the substructure.

For an axle load equal to Q = 30 t, the required thickness of the ballast is 25 cm and the required thickness of the sub-ballast is 15 cm for speed up to 100 km/h. However, a clean ballast layer with a thickness of 300 mm may prove to be the best solution.

The gravel should be of high hardness. Frequent monitoring of the ballast with the use of mechanical means and appropriate techniques is required.

The existence of high axle loads in combination with a weak subgrade requires that a sub ballast layer which is no less than 1 m thick be placed between the ballast and the subgrade.

4.1.5 Construction principles of the formation layer

The introduction of an axle load of 30t requires the provision of a formation layer of sufficient thickness in order to improve the bearing capacity just below the sub-ballast. It is obvious that a weak subgrade will lead to a rapid deterioration of the track's geometry, which will render the operation of heavy axle-load trains unsafe, thereby imposing an additional requirement for increased and more frequent maintenance.

For heavy haul rail transport the following are required:

• Stabilization of the substructure's soil with suitable mechanical means during construction

• Improvement of the foundation soil in case it is considered to be of poor quality


FIG. 2 Influence of the parameter db on the total thickness of ballast and sub-ballast layers ebt. Investigation of the impact of traffic composition on the economic profitability of a railway corridor - Fundamental principles and mathematical simulation for the selection of operational scenario for a railway corridor, PhD thesis (in Greek), Aristotle University of Thessaloniki, Thessaloniki, Greece; Pyrgidis, C. and Christogiannis, E. 2012, The problem of the presence of passenger and freight trains on the same track, International Congress TRA (Transport Research Arena) 2012, 'Sustainable mobility through innovation', 23-26 April 2012, Athens; Pyrgidis, C. and Christogiannis, E. 2011, The problem of the presence of passenger and freight trains on the same track and their impact on the profitability of the railway companies, 9th World Congress on Railway Research 'Meeting the challenges for future mobility', 22-26 May 2011, Lille, France.)

4.1.6 Dimensioning of bridges

Bridges require special design for the case of heavy haul rail transport. Since sometimes cracks are developed on the concrete along the bridges it is necessary that the substructure of the bridge be strengthened.

4.1.7 Dimensioning of the signaling system

The heavy haul rail transport results in an increase of the total weight of the train, thus increasing significantly the braking distance (which depends on the total weight of the train, the speed of the train, the total resistance of the train and the longitudinal gradient of the line). For this reason there is an urgent need for a special study of the signaling system.

4.2 Effects on the rolling stock

The increase of the axle load results in

• An increase of the static gauge of the rolling stock

• An increase of the train's movement resistance

The effect of the rolling stock's static gauge on the components of the railway system mainly concerns the required distance between track centers, the height clearance from the civil engineering structures and the geometrical adequacy of tunnels.

The increase of the train's movement resistance results in an increase of the required engine power of the locomotives. Therefore, special rolling stock is required for the heavy haul rail transport.

More specifically:

• The use of vehicles that are made by a material of increased strength is required.

The vehicles' design must be characterized by a constant effort to increase transport capacity. At the same time, it must be lightweight in order to be able to move at steep gradients and to develop higher speeds on straight paths.

• As an alternative strategy, the use of wagons with high payload in comparison with their tare (high payload - tare rate) and the increase of the number of axles can also be considered. A variation of the dimensions of railway vehicles and more specifically the reduction of the wheels' diameter is essential.

• The use of 3-axle bogies increases the transport capacity, keeping the forces applied on the track within the permissible limits.

Special probes for the impact of high load on the wheels (wheel impact load detector - WILD) can provide control and monitoring capabilities regarding the effects of heavy rail vehicles on the track, thereby constituting a valuable tool for the study of a heavy haul rail system.

4.3 Effects on the operation

The heavy haul rail transport regards trains of large length, thereby reducing the track capacity.

In addition, the heavy haul rail transport usually concerns the transporting of goods over long distances, that is, it is implemented at lines that are of long length. A typical example is Trans-Siberian line, with a length of 9,244 km, which crosses two continents, namely Europe and Asia.

The distributed power, or other words the use of more than one locomotive in the train formation (at the front and at the back, at the front and at the middle, and at the front, at the middle and at the back), make the transport operation much more effective. This option reduces stress on couplers and buffers, enables longer and heavier trains, improves force distribution in curves and enables quicker brake response.

The increase of the axle loads from 20 to 30t is expected to result in an increase in the track's maintenance cost by 3 times, depending on the quality of the infrastructure. It is still unclear as studies show that after the rails are subject to grinding processes and lubrication, the observed increase in maintenance cost is only 3%.

According to the literature , in case of two railway tracks with axle-load values of Q = 16 t and Q' = 22.5 t, respectively, and assuming that the speed is equal in both cases, the following mathematical equation applies:

(eq.3)

where:

Co, ' C: o the respective maintenance cost a, ß: coefficients that are empirically determined, depending on the type of the superstructure wear The ratio ' C/C oo is calculated between 1.41 and 2.78 (the ratio ß/a is calculated between 1 and 3.5), that is, the maintenance cost in the case of a track for which Q' = 22.5 t is between 41% and 178% greater than the respective cost for a track for which Q = 16 t.

5. ECONOMIC EFFICIENCY OF HEAVY HAUL RAIL TRANSPORT

A question faced by many railway operators nowadays is: 'What is more economically efficient for a railway company? Operating of conventional load freight trains or heavy haul freight trains on a new railway corridor which is dedicated for freight operation?' The issue of economic viability of rail networks that are used for the heavy haul transport over long distances contains several uncertain factors which affect the cost. The increased axle load initially seems as a profitable factor; however, a more realistic assessment is deemed necessary. The increased costs caused by the increased energy consumption, the increased investment in rolling stock, the requirements for the components of the infrastructure, the more frequent and increased wear of the track and the rolling stock constitute factors that may change the facts.

A technological solution is to increase the maximum allowable payload for a given axle load by improving the net percentage of tare.

Moreover, the use of multiple axle vehicles, which is more popular for road transport, could be a solution for railways as it increases the transport capacity. On the contrary, the strength and safety of bridges should be examined from scratch.

Mathematical models for decision-making regarding the transfer from a conventional load freight system to heavy load system have been developed. In a survey that was carried out recently, the economic efficiency of heavy haul rail transport rail (axle load of 30 t) was compared with the economic efficiency of rail freight transport of conventional loads (axle load 22.5t) with the aid of mathematical models. The comparison concerns the implementation and operation of a new single track of standard gauge dedicated for freight traffic and is implemented by considering various values of freight workload demand (10,000-130,000t per day per direction) and of connection lengths (S = 500 and 1,000 km).

Within the framework of this research, the rail infrastructure manager is also the owner of the rolling stock and the operating company. The financial indicator that has been considered to characterize the economic profitability of the new railway corridor is the net present value (NPV) of the investment.

The following steps were methodologically followed:

• Initially, a minimum daily freight load value was considered to be equal to 10,000 t per direction for both corridors. It was considered that this demand

• As concerns the conventional network, is served by 10 trains per direction which are formed of a number of locomotives and a number of wagons that can meet the above requirement. All wagons have an occupancy ratio of 80%.

• As concerns the heavy haul corridor, is also served by 10 trains per direction which are formed of a number of locomotives and a number of wagons and can meet the above demands given the same occupancy ratio (80%).

In both operation scenarios, a necessary prerequisite is that, in accordance with the UIC method, the track capacity saturation ratio should not exceed 70%. Assuming a connection length equal to S = 500 and 1,000 km, the NPV is thus estimated for both systems.

• The value of the daily freight load increased by 100% (20,000t) for both corridors.

Thus in order to meet this demand is was considered that:

• As concerns the conventional rail corridor, the number of wagons is initially increased (maximum value of 28 wagons) and thereafter, if demand cannot thus be met, the number of scheduled trains is increased. The occupancy ratio of the wagons remains constant and the track capacity saturation ratio does not exceed 70% of the practical capacity of the line, in accordance with the UIC method. In each case, the number of locomotives required is calculated.

• As concerns the heavy haul rail corridor, the number of wagons is initially increased (maximum value of 85 wagons) and thereafter, if demand cannot thus be met, the number of scheduled trains is increased. The occupancy ratio of the wagons remains constant and the track capacity saturation ratio does not exceed 70% of the practical capacity of the line. In each case, the number of locomotives required is calculated.

Assuming the connection length to be equal to S = 500 and 1,000 km, the NPV is thus calculated for both corridors

• The value of the daily freight load is gradually increased by steps of 10,000t, and the same procedure is repeated.

• After being suitably recorded, the results are compared and evaluated.

Table 3 indicatively presents, for a connection length of S = 1,000 km and for the different freight volume values under examination, for both exploitation scenarios:

• The formation of the train (locomotive and wagons)

• The number of daily services per direction

• The saturation ratio of track capacity

• The NPV for each of the two scenarios being compared

[coming soon] Table 3 Application of the mathematical model - results for connection length S = 1,000 km

It is noted that the initials EXCA (EXceeded CApacity) indicate that 70% of the practical capacity of the track is exceeded and, for this reason, the financial indicator is not recorded.

The diagram in FIG. 3 shows the change in NPV in relation to freight volume demand for both operation scenarios examined and for both connection lengths considered.

By examining all the combinations of demand and connection length, the following conclusions have been reached:

• The conventional load freight-dedicated corridor can serve up to around 40,000t daily for each direction, while the heavy haul rail corridor can cater for roughly three times that volume.

• Both systems have a negative NPV for a daily freight of up to approximately 20,000t for each direction.

• For daily freights per direction of up to 40,000t, which can be served by both systems, conventional load corridors are economically more profitable.

• For heavy haul rail corridors with a daily freight greater than around 30,000t, the increase in the connection length results in a significant increase in profitability as it translates to an approximate doubling of the NPV. Similar conclusions also apply for conventional freights; however, the point where it becomes profitable is at around 25,000 t.

The histogram in FIG. 4 presents the different cost incurred for the two exploitation scenarios examined, for daily freight volumes per direction equal to 30,000 t and for connection length S = 1,000 km.

FIG. 3 Variation of NPV in relation to the freight demand for a conventional axle-load line and for a line for heavy axle loads - length of connection S = 500 and 1,000 km.

FIG. 4 Construction, maintenance and operational cost for conventional and heavy axle-load lines - length of connection S = 1,000 km, demand for freight = 30,000 t per day per direction.

The intermediate calculations showed that in the case of the heavy haul rail corridor in comparison with the conventional freight corridor:

• The total construction cost of the infrastructure (studies, expropriations, civil engineering works, superstructure, substructure, track systems and facilities) is approximately 18.5% more.

• The construction cost of the superstructure is 15% more.

• The maintenance cost of the superstructure is about 52% more.

On the contrary, the cost for the maintenance of the rolling stock, the energy consumption cost and the personnel cost are lower.


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