Steps to reproduce:
1) Build a powered rail that steps up 15 vertical blocks (all rail is powered)
2) Build a cart launcher at the bottom
3) Enter cart
4) Launch cart
Result: Cart will travel up the incline for 15 blocks and then reverse direction
Expected result: Cart should continue up and jump off incline, or if track levels out, it should continue in the same direction.
Tested in Overworld and Nether on Mid-2009 MacBook Pro OS X 10.9.2