1. DISTINCTION BETWEEN HIGH SPEEDS AND CONVENTIONAL SPEEDS
There is no explicit definition of the term high-speed rail.
A speed of V = 200 km/h was initially established as a limit of distinction between a train running at 'conventional' speed and a train running at 'high' speed. The main reasons for the adoption of the above limit were the following:
• In most upgraded tracks, the radii of curvature in the horizontal alignment of the
layout had been selected for maximum passage speed Vpmax = 200 km/h.
• Just over this speed, the impacts from the geometric track defects are intensified, while some of the train functions become troublesome, and need special handling (e.g., an increase of the braking distance and the aerodynamic resistances of the trains and the train driver's inability to identify indications of the trackside signaling).
In the Technical Specifications for Interoperability (TSIs), the Trans-European High-Speed Network lines, are classified into the following three categories:
• Category I: New tracks that are specially constructed for high speeds and are suitably equipped, so that a running speed of Vmax = 250 km/h can be reached.
In some sections of these tracks, where, for technical reasons, the maximum speed provided for the interoperable trains may not be reached, it is possible for a lower permissible track speed to be imposed.
• Category II: Existing tracks that are specially upgraded for high speeds, and suitably equipped so that running speeds in the region of Vmax = 200 km/h can be reached.
• Category III: Tracks that are specially upgraded for high speeds (Vmax = 200 km/h), with special specifications, due to the limitations/enforcements imposed by the landscape or the compulsory passage through the urban environment, resulting in speed adjustment, depending on the case.
A third categorization is proposed in the literature reference according to which, for the distinction of a conventional-speed line from a high-speed line to take place, two conditions are used, which should be satisfied simultaneously:
1. Maximum attainable running speed of trains Vmax = 200 km/h
2. Average running speed between two successive intermediate stations Var = 150 km/h
According to 2015 data, 21 railway networks worldwide and, more specifically, the networks of China, Spain, Japan, France, Italy, Germany, Turkey, South Korea, Taiwan, Belgium, Netherlands, Russia, United Kingdom, Sweden, Switzerland, United States, Uzbekistan, Austria, Finland, Norway, and Portugal have at least one line that satisfies the above two conditions.
However the quality of a railway corridor's infrastructure, with regard to speed, depends on the value of the average running speed Var considering all track sections of the corridor.
High-speed intercity rail services usually serve distances of more than 400-500 km. For these trips, the intermediate stops are normally very few (from 0 to up to 2 stops).
Given the above, for a distinction between a conventional-speed line and a high-speed line, that is based on the total length of a corridor, one could use as the second criterion one of the following:
• The average running speed between two successive intermediate stops. But in this case the value of the distance between two successive intermediate stops Lst has to be near the average distance that is used in high-speed networks.
• The commercial speed across the corridor. In this case for Vc a minimum value of 150 km/h is proposed. This value is based on the competition between train and aero plane, in order to assure nearly equal travel times for the route.
Considering all the above, in this section, in order to distinguish a conventional-speed line from a high-speed line, the following three conditions that must be met simultaneously are used:
1. Maximum achievable train running speed: Vmax = 200 km/h
2. Average running speed between two successive intermediate stops: Var = 150 km/h
3. Minimum distance between the two successive stops in which the above average speed is achieved: Lst = 100 km
On the basis of 2015 data, the first 17 out of the 21 railway networks mentioned above have at least one line that satisfies the above three conditions.
Finally, the first 11 networks have lines with Vmax = 250 km/h and Var = 200 km/h. These networks can be characterized as 'very high-speed' networks.
2. HIGH-SPEED TRAIN ISSUES
The increase of speed beyond a specific value creates a series of issues and possible problems, the handling of which requires special interventions regarding both the rolling stock and the track.
The systematic operation of high-speed trains in the last 30 years allowed clear identification of these problems and in some cases the setting of crucial speed limits, beyond which they arise. The basic problems caused by the development of high speeds are the following:
• Increase of the train's aerodynamic resistance
• Problems arising in tunnels
• Dysfunction of the trackside signaling
• Increase of the braking distance of the train
• Requirements for high tractive power
• Lateral instability of vehicles in straight paths
• Special requirements in the track geometry, horizontal and vertical alignment of design
• Noise pollution of the surrounding environment
• Severity of damage in case of an accident (collision or derailment)
• Increase of vertical dynamic loads
• Decrease in the dynamic comfort of passengers
• Troublesome passage over switches and crossings
• Intensity of the aerodynamic effects and their impacts during the movement of trains in the 'open' track, and their passage from the station platforms It is characteristic that the above problems increase proportionally and non-linearly to the speed, resulting, beyond a specific limit, in the development of prohibitive conditions for the conventional railway.
The causes that will determine the maximum speed in the future, which may not be out performed by the wheel-rail system, should be sought in these problems (and mainly in the increase of the aerodynamic resistances and the braking distance).
The above problems are described and discussed in the following:
• Increase in the aerodynamic resistances of the train: The resistance Wm of a train
moving at a constant speed V on a straight path without longitudinal slopes is expressed by the following equation (the Davis equation):
(eqn. 1)
where Aw, Bw, Cw: parameters depending on the characteristics of the rolling stock.
The first term, Aw, is independent of the speed of the train and represents the rolling resistances. The second term, BwV, is proportional to the speed, and represents the various mechanical resistances (rotation of the axles, transmission of movement, etc.), as well as the air friction resistances along the train's lateral surface. The third term, CwV2, changes in proportion to the square of speed, and represents the aerodynamic resistances (aerodynamic drag).
FIG. 1 Change in the resistances of movement with respect to speed
V (TGV Train A, formation: 1 power vehicle + 10 trailer vehicles + 1
power vehicle).
According to FIG. 1, a speed change from 200 to 300 km/h results in a change of the aerodynamic resistance of the train by 100%, while the mechanical resistances remain literally unchanged. At high speeds, the aerodynamic resistances determine, therefore, the total resistance of the train, and hence, the required motor power of the power vehicles.
• Problems arising in tunnels: During the passage of a high-speed train (Vp = 200 km/h) through a tunnel, the following aerodynamic problems arise:
• Sudden change of pressure: The passage of a train through a tunnel is always accompanied by a fluctuation in the pressure exerted frontally and laterally to the train. This fluctuation is more annoying to the passengers, as the time, within which it takes place, gets less, and therefore, as the speed of the train's movement gets higher. The great differences in the pressure inside the tunnel, and also at its entry and exit, may cause earache and headache to the passengers.
Experimental tests have shown that, for speeds of more than 200 km/h, a notable reduction of the acoustic comfort of passengers is observed. It is mentioned indicatively that the TSI that relate to the construction and operation of the railway tunnels focus mainly on the maximum permissible change in pressure (delta_Pmax) generated inside the tunnels. In this context, it is required that the maximum change (delta_Pmax) in the pressure along an interoperable train should not exceed 1,000 Pa during the crossing of the tunnel, and for the maximum speed permitted by the specific civil engineering structure.
• High aerodynamic resistances: During the passage of a train through a tunnel, the aerodynamic resistances, for the same speed and the same train formation, are much higher than those generated at the surface sections of the track. The major impacts resulting from the increase in the aerodynamic resistances inside a tunnel are the increase of the exerted forces on the train, which result in greater energy consumption.
• Interaction of trains travelling in opposite directions: In case of a double-track tunnel, the crossing of trains travelling at speeds of more than 220 km/h may cause damage to the rolling stock (particularly breaking of window panes), due to the increased pressure waves that are generated.
'Tunnel boom', is a phenomenon of radiation of impulsive sound from the exit of a tunnel used by high-speed trains (wikipedia, 2015).
Upon the entrance of the train to the tunnel, shock waves are emitted from the inlet to the outer environment (entrance waves), while a similar effect is observed upon the exit of the train from the tunnel (exit waves).
• Dysfunction of the trackside signaling: When the speed of the train increases, the visual perception of the trackside signaling becomes increasingly difficult. In bad weather conditions (e.g., fog), the identification of signals at speeds just over 220 km/h is troublesome, if not impossible. Therefore, at high speeds, all the signaling systems that are based exclusively on the identification of signals by the train drivers are incompatible.
• Increase of the braking distance of the train: The braking distance of a train increases roughly in proportion to the square of speed. This fact, combined with the reduction of adhesion at high speeds, generates an extra increase in the power consumed, during the braking operation.
• Requirements for high tractive power: The required tractive power increases in pro portion to the third power of speed. The great nominal motor power required at high speeds, combined with the need of instant supply of great tractive powers to the net work, makes the electrification of trains a necessary condition for the development of high speeds. At this point, it should be mentioned that, at speeds V > 160 km/h, a discontinuity may be observed in the contact between pantograph/overhead power wires, resulting in problems concerning the electrification of trains.
• Instability in straight paths: The speed parameter involved in the expression and value of creep forces determines the lateral stability of the vehicles to a great extent. At lower speeds, the movement is stable. Over a specific speed, the movement becomes unstable, causing oscillations of high amplitude, contact of the wheel flanges with the rails and lateral forces that may cause lateral displacement of the track.
• Special track geometry alignment requirements: The following three individual problems are identified:
• High centrifugal forces in the horizontal alignment curves: During the movement of a railway vehicle in curves, centrifugal forces develop in curves, the value of which increases in proportion to the square of speed. To reduce these forces, it is required to adopt great curvature radii on the horizontal alignment, and apply high cant in high-speed lines. An overly high cant value causes problems to the coexistence of fast and slow trains in the same network.
• High vertical accelerations in the curved segments of the vertical alignment: During the passage of trains from the curved segments of the vertical alignment, the vertical accelerations increase in proportion to the square of speed.
• High impact of geometric track defects: Numerous measurements taken in various networks have shown that the impact of dynamic loads on the track superstructure increases in proportion to the speed and is directly proportional to the ride quality of the track, that is, the geometric track defects.
The geometric track defects comprise the main cause of additional dynamic stresses which are generated by the interaction track-rolling stock.
• Noise pollution of the surrounding environment: For speeds of up to 300 km/h, the noise level increase is a function of the third power of speed, while for higher speeds, the acoustic annoyance increases in proportion to the sixth power of speed.
• Severity of accidents: In case of collision between trains, or collision of trains with obstacles on the track, the material damage is definitely more severe and the likelihood of injuries is higher. The same also applies to the case of derailment.
• Increase of vertical dynamic loads: The increase of speed does not affect significantly the load change caused by the suspended masses of the vehicle (car body), since the vertical accelerations of the car body increase less quickly than the speed, and they may be restricted by reducing the natural frequency of the car body, or by ensuring a relatively good track quality.
In contrast, the semi-suspended masses of the vehicle (bogies), and particularly, the unsuspended masses (wheelsets) change significantly with the increase of speed, and increase the total value of the vertical dynamic load.
• Passage over switches and crossings: A good operation of high-speed network requires the passage of trains over switches and crossings with speeds higher than those applied in conventional networks. This requirement automatically generates new requirements in terms of the design and construction of switches and crossings.
• Reduction of the dynamic comfort of passengers: The increase of speeds automatically implies the increase of vertical and lateral accelerations of the car body, which have direct impact on the dynamic comfort of passengers.
• Intensity of the aerodynamic effects and their impact on the 'open' track and plat forms: In 'open' track sections, the following may occur:
• During the passage of a train at high speed, the pressure along the lateral surface of the train as well as in the area adjacent to it changes and vibrations may affect the residential window panes located near the railway track.
• During the crossing of trains travelling in opposite directions at high speed, the pressure distribution along the trains affects their dynamic behavior.
On platforms, the air flow field that is generated intensifies with speed and may cause the following:
• Loss of balance, difficulty in walking and passengers or staff on the platforms near the tracks to be pushed violently.
• Ballast turbulence with the risk of injuring people on the platforms and causing damage to the rolling stock.
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