Moving object down or up the spline using gravity

[velikizlivuk]

There is no gravity when moving objects along the spline. Can be achieved using input like in Rigidbody scene but I need the object to move by itself. 
How to detect angle of object moving on spline (that uses motor controller since I need to have option to move object using some interaction buttons) so I can add velocity according to slope of the spline?
Scenario: player moves train along spline (motor controller) but train has stopped on slope and player does not apply input. I need that train to move down the slope by itself since there are no breaks on train.