when you put coal in a furnace cart, it will go forward for about 5 seconds, and then stop. smoke still comes out of the cart, confirming that it should still be moving. i tested this on a track with no hills (simple circle about 5x5 blocks), and it still does this. adding more carts in front of the furnace cart does not affect the furnace cart's functionality (stops after 5 seconds of travel).

tested in both multiplayer and singleplayer. also test with creative's no-coal shortcut for powering furnace carts (simply right clicking without coal in creative).