At my work we make wheels for this application. They are cast aluminum with a urethane tire. Some have made them from BMX skyway wheels but they are not as durable. We worked with a local guy and developed the patterns, a local foundry pours the castings, we machine them and mold the tires onto the castings. You might try calling any of the rail bike operators they might sell old inventory. I think most of them use the multi seater rail car configuration but some of them might have bikes