Periodic timetabling for railway networks is usually modeled
by the Periodic Event Scheduling Problem (PESP).
This model permits to express many requirements that
practitioners impose on periodic railway timetables.
We discuss a requirement practitioners are asking for,
but which, so far, has not been the topic of mathematical
studies: the concept of symmetry.
Several motivations why symmetric
timetables might seem promising will be given.
Though, we provide examples proving suboptimality of
symmetric timetables, in general.
There are many obstacles to overcome when trying to introduce
symmetry into the graph model of the PESP.
Nevertheless, adding symmetry requirements to mixed-integer
programming formulations explicitly, enables MIP solvers,
such as CPLEX, to terminate earlier with good solutions.
