A Gated Rail would work as an entrance/exit for minecarts into/out of buildings. I was thinking of a fence-gate sitting on top of a rail. It opens/closes on redstone pulse.

Filtered Unloaders are upgraded Unloaders where you can specify a blacklist/whitelist of items which are considered. That could prevent unloading Rail Signals, in the case where the Leave 1 item in minecart slots config option is not applicable.