Few games have officially something like pak sets. Simutrans has essentially embraced the concept of total conversion mods and taken it to the level where you can't simply talk of Simutrans as a specific game, you also have to mention the pak set being used. Something that is a good idea in pak64 might be a bad idea in pak128 and vice versa. It is however not so extreme that Simutrans is simply a game engine, and the pak sets different games.
In additions, Simutrans has add-ons and skins, which are lesser level modifications. Add-ons work poorly or not at all with other pak sets than the one they are made for. Pak set graphics size must at least match.
And then there is the unfortunately named Simutrans Experimental, which is a project that has different ideas about how Simutrans should simulate. (And perhaps a few other things. I haven't paid much attention to it.) It is not a testing ground for new Simutrans features as some have thought. (Although some features that are unrelated to the fundamental differences between them might appear first in one, and the other later on.) It is rather a separate, though related, game, with its own pak sets.