(I will probably split trains api and actual trains into two mods, to allow for extensions to be enabled individually)
At the moment, you can try around with the trains. There are some debug messages that shouldn't disturb you. Note that anything may change in future releases.
- place locomotives or wagons by picking the item and placing it on a track.
- right-click a wagon or locomotive to sit onto it.
- inside a locomotive, use W/S to accelerate/decelerate the train. This will fail if the train can't move in the desired direction. Shift stops the train, aux1 (default E) or right-click on wagon will let you off.
- drive two trains together and they will connect by right-clicking that green icon that appears.
- punch the red couple icon between wagons to discouple them