Oh so those were actually dae models and not interiors? I that would explain that hole you've got in the platform too.
Yep. They're all dae models I put together in milkshape (which I'm still new to).
I plan to go back and make them... more interesting.
However; the bridges and ramps are sketch objects which can be created in the mission editor and are basically rectangular prisms you can resize and map materials to. There's some other functionality with it but I haven't experimented much.
Also: the texture on that hole rotates. It's sort of cool.