i'm attaching a minecraft screen capture and a DxDiag with all the details.
how to reproduce
1- make a T shape
2- make sure there's a curve into the T
3- put a cart and push it
3b- put a cart on a powered rail and power then push it
3c- put a cart on a powered rail in a slope and power and then push it
4- see the cart go to the intersection and either lose all the speed, derail or turn around completely