Hi,
The modelization of friction, due to climbing up a slope, in the physics engine doesn't seem right in some circumstances. I think the friction decreases too slowly after we are back on flat ground. It takes a full 5 tiles to go back to the default value for frcition, and makes the early petrol engines virtually useless (except maybe in an hypothetical PakBelgium :D). But that's just my opinion, the following is probably a bug:
If a vehicle stops but had left-over friction, it'll still have that friction when it starts again. Even if it did a U-turn in the meantime. See screenshots here: http://imgur.com/a/6VSIB
I have a feeling that the "persistence of friction after a stop" bug also occurs with sharp corners as well as slopes -- though this is just a hunch, and I'd have to test properly in order to verify this.
I'm not a mechanical engineer, but thinking about this a bit more, I don't see why friction should persist at all instead of just dropping back to the default value as soon as the slope/turn is over. It shouldn't be harder to accelerate on flat ground just after a hill than to accelerate on flat ground in any other situation.
Ahh, this is not an entirely straightforward topic. Because hills in Simutrans are very sudden, to make realistic (or realistic-ish) hill climbing characteristics, I have to spread out the effect of the hill over neighbouring tiles, or else vehicles, trains in particular, would find hills an insufficient obstacle. Things do get very complicated when there are stops and in particular u-turns, but, for consistency, one has to imagine a stop next to a hill as a stop that is still on the hill. This is not ideal, I appreciate, but it seems unavoidable given the limitations of Simutrans's landscape.
However, it is not long before we will get two rather than one steepnesses of hill in Simutrans. The new, shallower type of hill can be set to have not only less resistance but also its resistance spread over fewer tiles. Unless anyone comes up with anything ingenious, that might be the best that we can do if we are prioritising - as we are - economic realism.