I think a train which is too heavy for the brakes should be punsished in some way when attempting to go down a hill that is too steep. Basically this would work in the same way as the realistic acceleration model, but just in the inverse direction. For simplification one could say, the engine(s) do(es) not have to pull the train down the hill, but push against it in order to make it slower. And if the tractive effort of the engine(s) isn't high enough, it cannot brake the train. But what should happen if a train is too heavy? In reality it would speed up (and perhaps derail). In the game, one could perhaps just make it break down or slow it down to 1mph (for safety reasons
![Wink ;-)](./images/smilies/icon_wink.gif)
Admittedly, this is just about realism, for the average player this may not a useful feature and I do not know how difficult it would be to implement it, but I felt like posting the idea anyway.