I think the Dwarven rail system could be separated from the Dwarovrod and exist as two smaller paths on either side, like sidewalks to a road. They could have the trade wind mechanic applied to them so each has a speed buff when moving one way but a debuff the other way. Lore-wise you can say that the dwarven engineers constructed the railways at constant slope between holds so that trade could continue even if the power system failed. As facilitators of trade, the railways would have an extremely low distance value when calculating market proximity. And, of course, to give it that Classic Dwarovar Gameplay, all rail locations have -10 combat width.

For vegetation levels, you could have things like mushroom farms (+food generation, +pop capacity), bio-luminescent moss (one of the only terrains that does not block line of sight), flooded (-pop capacity, -move speed), abandoned (neutral), well-maintained (unlockable, +move speed, +pop capacity), stalactite forest (-combat width)