On the Complexity of Train Assignment Problems

We consider a problem faced by train companies: How can trains be assigned to satisfy scheduled routes in a cost efficient way? Currently, many railway companies create solutions by hand, a timeconsuming task which is too slow for interaction with the schedule creators. Further, it is difficult to measure how efficient the manual solutions are. We consider several variants of the problem. For some, we give efficient methods to solve them optimally, while for others, we prove hardness results and propose approximation algorithms.