To recreate the bug:
1- Create a minecart track which goes up all made up of powered rails. Make it above 15 block high for it to really work.
2- Lunch a minecart so it would climb up...
but it will stop at 14 block high for an empty minecart and 7 block high for a minecraft with chest or minecart with a zombie riding it.

If a minecart goes to fast before reaching a a rail who goes up. It won't climb it at all.

First image show the minecraft track.
On the second image the pink wool shows the height a empty minecraft reach, and the green one the height the other minecraft can reach.