What I expected to happen was...:
A minecart on a rail should travel through a portal and continue traveling into the overworld or nether if there is another rail on the other side, linking the dimensions.  This breaks automatic item transport between dimensions, chunk loading and moving mobs with rails and minecarts.

What actually happened was...:
The minecart will travel through the portal, but simply gets stuck inside the portal frame and does not stay on a rail between dimensions.

Steps to Reproduce:
1. Place a loop of rails at the same y value as the nether portal in both dimensions
2. power the rails and send minecart through
3. minecart won't come back and won't move in the next dimension.