When Rail Explorers ran in Saranac Lake, they typically had a single guide riding a tandem bike at each end of the group, so they can definitely be propelled by a single person. Obviously, a lot will depend on the rider's physical condition, the grades involved, and the design of the bike itself. I have not ridden any of the current operations, and the North Creek operation uses bikes of a different design than Rail Explorers, so I can't offer any guidance to you. It might also be possible for a single rider to pair up with one of the guides so that they don't have to do it alone. I would just ask them when you book your ride.