The runaway train.....

[HBBahn]

Could there be something mechanical in the loco causing extra friction when travelling forward? This might explain what's happening, but I doubt it.
If you switch the green and yellow wires to the motor, it will 'hard wire' a reversal of polarity, so forward will be reverse. If you tried this, it would rule in or out a mechanical fault. ie. if you now have the same problem, but in reverse, it's something in the electronics.
(edited for wire colours)

 

[HBBahn]

PS. if you have inclines on your railway, and have load compensation switched off, you will eventually have a genuine runaway train. Trust me. Watching a couple of grands' worth of RhB stock disappear off a two foot cliff one after the other is not something to be experienced if you can avoid it.