The concept of families would be a series of image variants of a given tile which would be included in the same "family". Simutrans would pick one of the tiles in a family to display, randomly.
The point of this is to add more variation to the terrain and ways, as families of several slightly different tiles could be made (say, for grass), and in the final effect we'd have a more varied landscape.
All of this is just IMHO:
Good idea! However, for many object types you can simply drop in more types and have them randomly selected. So, this makes sense mostly in cases, where there is one logical object needed - usually because of player interaction. But on the other hand, player should not even wish to meddle with the random selection, to keep the additional complexity outside game interactions. That leaves only a handful of niches, and you certainly got them right I believe :)
1) Ways are a good candidate.
2) I'm not sure how much advantage is there for grounds over ground-objects, considering the memory spent eventually for n^2 requirements (textures*maps).
3) Randomized loaded vehicle graphics would absolutely rock in a few niches, such as steel, cars, machinery etc.
4) Buildings are a no-no, already randomized per type or player can strongly wish to control them.
I'd love to have this possibility, but then again, I have many dreams... Support, but without any false hopes ;D
This have been in there in the old tile system and is now emulated by the groundobj (which are essentially the four old tiles: stones, flowers, moles, and bushes).
Experimental is about to do something slightly related for vehicles, in the form of liveries.