Currently it is possible to ride Minecarts through portals by letting them derail and then drop into the Portal. However this works only for the way down (although it is surely possible to do some fancy piston contraptions for the way up).

So if the rails could actually connect with the portal, the transfer would work seamlessly in both directions.

For implementation probably a special Rail-portal block is required that automatically spawns instead of a regular portal when there is a rail block on the other side.

implemented by brute force: simply fixing adjacent rail orientations on portal block update