If you need to rely on the customer vehicles themselves for any part of the train's operation (wheels touching the ground, coupling, electrical connections...),

If you need to rely on the customer vehicles themselves for any part of the train's operation (wheels touching the ground, coupling, electrical connections...), it's going to be extremely difficult. Railways have strict requirements for inspections, etc. of customer cars—one malfunctioning car can be catastrophic (if a coupler malfunctions and lets go, for instance). If you're not relying on the customer vehicles for any part of the operation, you're basically describing existing auto-rail operations. They're not extremely common, probably because they're a terrible waste of space compared to traditional passenger rail cars. Our goal is to move people and goods around—not extra vehicles.