It's all in the cards!
Henzell says...
  1. Acts like one-way stair to another location on the same level. Use either > or < to enter. Adjacent hostiles and allies will travel with you to the other side. Transporters are otherwise like normal floor tiles that can be safely traveled over and can hold monsters and items. Monsters can't use transporters by themselves.
  2. Added in 0.20, replacing the old {teleporter} terrain feature.

Information based on master branch and the great LearnDB.
© Dungeon Crawl Stone Soup, 2010.