MacOSX 10.10.5 (14F27), Java 1.8.0 64Bit
Minecarts no longer gain momentum from the user pressing the forward or backwards keys.
Steps to replicate:
Open minecraft version 15w43c.
Create new a creative world with default settings.
Place at least 10 rails with a minecart on it.
Enter mineart and press 'w' or 's' (forward/backwards keys)
Observe minecarts do not gain momentum from key presses.
This can be replicated every time.