how come (according to MCEdit) stairs and slabs from railcraft (9.12.2.0) are tile entities and not blocks? like they are in vanilla?

Because otherwise I'd need like 400 block ids.

And the slabs can actually have different top materials from the bottom.

is that a bad thing to have that many ids?

thats actually quite cool about the slabs, i didnt even know that. usually its not possible for slabs to stack like that.

When I added then we only had 256 ids total, including vanilla stuff. It's better today with 4k, but there are mod packs out there that even hit that.

It's not like it really matters, they don't tick so they have almost zero extra overhead.