In the old days, Colorado simply shipped prisoners to the State Penitentiary in a full set of irons chained to whatever transport was available, or chained up in the baggage car with a couple of large deputies as company. When buses came along, Colorado switched to specially equipped buses and vans to haul inmates to prison, which they still use today.
I suspect that if there were a strong enough local need, a particular line would simply "bash" one up out of a spare box car, or perhaps a passenger car that had seen better days and was retired from service, which would at least give them some sort of heating and plumbing to begin with.
I can't see too many smaller outfits contracting one out unless the particular state or the government were willing to invest in it, due to the cost of such a special purpose car and the question of recouping the cost through sufficient "passenger" revenue". Just my opinion, though.
It would make a terrific project, especially for one of the older lines from the early part of the 20th century! :thumb: