Support train randomization for NPC trains

[custodian]

Summary

All spawned NPC trains use same model as selected by player.

Proposal

Allow any train from the repo to be spawned, if length of the train does not exceed minimal station length on the route.

Alternatives

Introduce scenario settings to specify minimum and maximum allowed train length to be spawned with scenario routes
Introduce scenario setting which can explicitly allow to spawn long trains (exceeding some of the stations lengths). In this case NPC train should stop, do not open doors, wait if required and proceed to the next route point.

Additional information

Trains from user Mods should be compatible by default with any scenario, so only train length should matter in this case.

Can be easily checked / implemented after #520