By fixing the average speed between stops, the timetable has a decisive influence on energy consumption. The design of the timetable underlies rigid requirements imposed by

• Technology: installed power
• Safety: speed limits
• Service quality: fast transportation, short travelling time, punctuality
• Capacity and mixed operation: no interference with other trains running on the same line.

Within this rigid framework, some degrees of freedom remain which can be exploited to optimise the timetable for energy efficiency:

• The amount of buffer times foreseen by a given timetable is of crucial relevance for the realization of energy efficient driving strategies. Elasticity of average energy consumption with respect to buffer times is very high, i.e. slightly increased buffer times lead to strong reductions in energy consumption, especially if original buffer times were low (<5% with respect to shortest time driving strategy). Buffer times are also a key factor for punctuality and surveys demonstrate that most passengers give higher importance to punctuality than to minimum reductions in travel time. As a consequence, there is optimisation potential for both energy efficiency and service quality.
• On many lines there exist low-speed sections that could be removed without major costs. This would not only reduce travel time but also reduce energy consumption since the deceleration and subsequent acceleration caused by speed limits on short parts of the line usually overcompensate the energetic effect of reduced air drag in speed limit sections.
• Average energy consumption can often be reduced by reassigning running times between intermediate stations while keeping the running time between main stations constant. This can either aim at the strategy of consciously delayed trains or at a homogeneous distribution of buffer times to the different parts of the line. Due to the non-linear dependence of energy efficiency potential on buffer times, a reassignment of the available time surplus will often have beneficial results. However, the effect of this measure is expected to be rather limited.

Depending on the optimisation strategy win-win situations can occur:

• Increased buffer times increase running time only slightly but may improve overall punctuality considerably.
• The elimination of low-speed sections reduces running time

Other priorities

Timetable design is a highly complex task in which numerous restrictions and constraints have to be taken into account. Energy efficiency obviously has a low priority in this context.

Timetable optimiser (TTO) as part of the Metromiser

For details cf. DAS in suburban operation