Fix: trains cancelling their pending reversal when ordered to go to a depot behind them

Previously, if a train had been ordered to reverse, and while it was slowing down, was
ordered to travel to a depot that is behind it, the train would continue forwards.
Also when a train had been ordered to reverse, and while it was slowing down, was
ordered to travel to a depot that is in front of it, the train would not cancel the
reversal.
In both cases the train would travel away from the target depot.

Trains in this situation now behave correctly and will travel towards the depot.