When you right click a furnace minecart with a bucket of lava it inches forward. Lava used to work as a fuel source, last time I tested (before powered rails) it still worked. If lava bucket minecart fuel was removed -> Spamming right click moves furnace carts forward(even using your fists).
Comments 2
Feature Requests/Suggestions - http://goo.gl/7qGlN
At present, this is the intended behaviour. If you wish, you can post a proposal for lava powering minecarts on the MC forums.