Okay my train system is more than a two years old now, so there are several possible problems:

- handbrake on the train or carriage is active
- Keen updated the wheel behavior (again), so the settings of my trains and carriages don't fit anymore. ( I hate when they do this, it always ruins half of my builds)
- suspension height is wrong/reset after an update

I don't have SE installed at the moment so I can't look into the game right now but you can try reducing the friction and make sure the lower suspensions (the ones on the rail) are raised to the max.

I can only get this one up to 30km downhill. is that normal? going forward on flat track I can only get around 15km an hour