Is the Bangor Abbey Street Park and Ride useful for train passengers?

Yes.

The official Park and Ride listing identifies Bangor bus and train station on Abbey Street as serving both bus and rail. It is a different location from the smaller Bangor Dufferin Avenue bus Park and Ride. Abbey Street is the stronger starting point if you need the railway towards Belfast rather than a particular bus. Choose the station entrance in your satnav, allow time to walk from the car to the platform, and verify any parking restrictions displayed locally.