Minecarts not reversing down slopes

Preamble:
A villager in a minecart gets pushed up a slope which terminates in a solid wall. (In this case, collision with another minecart a wee bit down the track)

What I expected: Gravity pulls the minecart back down the track.

What happened: Minecart jams at the top of the slope. See attached pic 2014-08-19_22.26.16.png.