Usually a transport car is an older patrol car taken out of daily heavy-duty service and "retired" to "easy duty" of prisoner transport. Some will still have complete markings and emergency lighting, but roof lights and traffic lighting is sometimes removed as it isn't needed. They will always have full communications. Often they will have additional security and restraint features for prisoner transport. Transport vehicles are sometimes marked as such as transport officers do not stop when someone tries to flag them down, the "corrections" "jail services" "transport services" lettering lets the person can see it's not a patrol vehicle.
Sidebar: The transport officers with a prisoner do not stop - they will contact dispatch for a patrol car to respond to the person attempting to flag them down.
There was nothing more frustrating than transporting a wild out-of-his-mind, stinky, kicking, spitting prisoner and having cars in front of you slowing to 5 MPH below the speed limit because they see a LE vehicle in their rear view mirror. "Get out of the way!"