It seems to have be used for planes, trains and cars so far, for a variety of different reasons. Maybe we just should not use it? or give it a specific definition?
It could be by itself, but in the context of the question and other tags I think it is acceptable. For example, you may be searching about transportation by airplane, so the airplane tag may not be sufficient in filtering down the results, but searching for questions that are tagged both airplane and transportation is very helpful.