The situation in freight operation

The driving pattern e.i. the speed over time diagram has a considerable influence on the energy consumed by a train on a given trip. Whereas in passenger operation this pattern is mainly governed by timetable, stops at stations and speed limits, the situation in freight operation is quite different. Most freight trains do not have to obey strict time schedules but rather stay within certain time windows. On the one hand, this leaves more room for the energetic optimisation of the driving pattern. On the other hand, in mixed infrastructures passenger trains usually have priority over freight trains which leads to frequent unscheduled stops for freight trains. These unexpected stops impede the operation of a DAS.

Low-density networks

Railways with low traffic density and/or homogeneous freight operation therefore offer the greatest potential for freight DAS. This is the case in countries like the US or Australia, but also on individual lines in Europe. In such cases, DAS does not aim at coasting only but rather on an optimised running pattern for the whole trip.

Dense and or mixed networks

In cases where freight operation has a high density and or shares the infrastructure with passenger trains, DAS can only operate in an effective way, if the system takes into account the traffic situation. This requires a radio (or other) link to the control centre level.

Components of a DAS

Just as in passenger operation, DAS for freight operation require the following on-board components:

• Storage medium storing all the relevant data for an individual trip (infrastructure data, vehicle data)
• Information system monitoring driving time and train position
• Computer unit using the above data to determine driving strategies and display them to the driver

In order for freight DAS to operate in a reliable manner several issues have to be resolved:

Train positioning

The exact train position is essential for the calculation of driving recommendations. On main lines a precision of 100 m is usually sufficient.

• In suburban DAS the wheel impulse counters (odometers) are sufficient since frequent stops at stations can be used to compensate slippage-induced errors. This method alone is usually not sufficient in main line operation since stops are too rare to minimise this inaccuracy.
• Therefore additional satellite-based positioning (GPS, Galileo) has to be used: This method usually has an accuracy of 5 m, but due to signal reflections on metal surfaces along the track (multipath propagation) there may be exceptional errors of up to several hundred meters. In addition the positioning signal may be interrupted in tunnels or stations.
• Combined positioning (by sensor fusion): The most reliable method is a combination of the two methods. If the resulting position data differ, a so called sensor fusion calculates a weighted combination of the two. A field test of this system realised in Switzerland (cf. Meyer et al. 2002) yielded only in less than 5% of all cases an error exceeding 10 meters.

Data supply

A DAS requires different classes of data to be updated in different time intervals:

• Permanent data: Vehicle data
• Long-term data: Track data base (to be updated annually)
• Mid-term data: Time table
• Short-term data: Data on temporary low-speed sections (to be updated daily or even in real-time). This includes train configuration and vehicle mass which vary permanently in freight operation.

Systemic optimisation

Driving advice systems for freight operation in dense networks are only effective if a link to the control centre is provided to take into account the traffic situation. This would require some communication channel between the DAS and the control centre as well as a corresponding IT support at the control centre.

Extended functionalities

Future driving advice systems could go far beyond giving driving recommendations and offer a wide range of functionalities such as monitoring for maintenance processes and accident investigation etc.

Capacity

DAS could help drivers to stay inside their infrastructure slot at all times. This would improve slot management and thus increase capacity.

Wear

Coasting instead of braking reduces the wear of the brakes especially if electric braking is not possible.

Apart from the barriers for DAS in general described in detail in DAS for main line operation, in freight operation additional hurdles arise:

Unscheduled stops

In mixed networks, freight trains are frequently confronted with "train conflicts" due to higher priority of passenger trains. Such incidents make an effective use of DAS impossible. Therefore, as long as no link to control centres is realised, the applicability of DAS for freight operation is limited to situations with low traffic density or dedicated freight lines.

Variability of vehicle data

Whereas in passenger operation the vehicle mass and aerodynamics remain virtually constant (as long as train configuration is left unchanged), the mass and running properties of freight trains are subject to frequent changes: cars are added or removed from the train, cars are filled or emptied at freight centres etc. This leads to a high variability of train dynamics. However, these dynamics are a vital input for DAS calculations.

At DB AG a tool has been developed to solve this problem. "ESF Lok" is an on-board tool to determine the vehicle dynamics in real-time. ESF Lok calculates the running resistance and the mass from the acceleration the train shows as a reaction to an applied traction force. The acceleration is determined by GPS and odometer and the traction force is measured by the locomotive. However, the latter feature is provided only by certain locomotives, such as the series 101 at DB AG. It is not clear whether such a system can be generalised to all stock.

The saving potential to be achieved by driving advice systems in freight operation cannot be given in a general fashion due to the strong dependence on the following factors:

• Traffic density and degree of demixing of the network
• Type of freight operation (transporting mixed goods with frequent stops vs. trains with homogeneous mass goods on A-to-B relations with no intermediate stops)
• Integration of control centre level to take into account traffic situation

For most European railways the saving potential of current driving advice systems is expected to be well below 5%. This could change dramatically as soon as an integration of the control centre level is achieved.

In principle, coasting and other energy efficient driving strategies are very promising for freight trains, since the potential for regenerative braking in freight operation is very limited.