Thanks for the responses guys.
Aabeck, a prox sensor on the end of the hose may not be able to see the railcar and we would need a target mounted to the fixed inlet, as parky mentioned. I can provide a picture once it's setup.
The prox switch is a great idea but I should have mentioned that this hose will not be permanently attached to the building and may be removed periodically. There are 2 fixed connections on the building and 2 on the railcar, and a hose for each side to connect the building connections to the railcar connections.
The prox could be a viable solution, but I think we would need a connector for the sensor cable at the building that the operator or maintenance can detach when they need to service the hose, and already I question the longevity of that setup.
Both hoses need to be connected at the same time to start the system, so if we were able to sense a complete circuit between the "out" hose and the "in" hose, essentially using the railcar as a conductor, that would be ideal. There is electrical continuity between the hose ends. I feel this could be done, but not sure how to go about it safely.