I've played with trains for while now and I noticed that even if it's driving down a slope it still consumes 25MW. Since fics-it is all about efficiency it doesn't make sense. I would suggest that breaking train should produce electricity or at least it shouldn't consume it.
Another idea: when you're driving full speed and reach the end of tracks, you just stop instantly. I think that this should create a "power production" spike which would kill the grid just like not enough "power capacity" does. At least you would now have a reason to break and the conservation of energy would work.