Pipes

Pipes are pretty straightforward objects with the first param being their height.

example pipe: 42 y x 0x18 -- a vertical pipe with height 3 (including the lip), and not transportable

transports

Pipes come in two flavors: transportable and non-transportable. Which object id you use decides this. For example, 0x18 is a non-transportable vertical pipe, while 0x17 is the transportable version.

non-transportable pipes are just physical objects in the room. Mario can stand on them, but not warp by going down in them.

transportable pipes enable Mario to warp. It is essential that a transportable pipe have a corresponding transport defined in the room's data, otherwise Mario will warp down into a black void and soft lock the game (just like doors).

vertical vs horizontal

vertical pipes are in practically every object set which makes sense as almost every Mario level ever has vertical pipes in it.

But horizontal pipes are available in far fewer object sets.

double ended

vertical and horizontal both come in double-ended variants. This is just a different object id. Each variant has the same object sets as its corresponding type.

airship pipes

These are pipes with a narrower lip and are always silver. So far haven't explored them much.