I have always thought that underground belts & pipes were cool, and thought train bridges\tunnels that would work over certain link lengths as with underground belts & pipes would be cool, but what would be the coolest, would be to allow train traffic in and through Factorissimo(version)... reducing the size of a depot\compound\outpost. This can be different buildings that also come in multiple sizes.

I think that would be cool too, but Factorio's modding API does not support such things at the moment.