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.
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