Minecart with Monster Spawner

A Minecart with Spawner will probably spawn mobs as it goes along its respective rail. A possibility is that it will spawn mobs when it hits a Detector Rail, Activator Rail or Powered Rail, or simply spawn mobs constantly like a normal Spawner.

This also gives the intention that they might add Monster Spawners to survival mode, as being craftable. Though another likely explanation is that they could spawn in Abandoned Mineshafts or maybe both.