Prisoners do not wear diapers when being transported from prison to prison or from a jail to a prison.
However there may be an exception that a prisoner wear a diaper when being transported if they are incontinent and have bladder or bowel issues and cannot hold it in.
Although then the prisoner would likely already be wearing diapers for their incontinence and would be wearing diapers during transport.
However than that a prisoner is expected to use the toilet before the transport and then they will be expected to hold in their pee or poop until they arrive at the prison.
For security reasons the prisoner would not be allowed out of the transport vehicle to use the toilet at any rest areas.
A large prison bus may actually have a toilet on it though but small prison transport vans for transport of prisoners do not have toilets.
Some prisoners have actually peed and pooped their pants on their way to a prison during long haul transports and most of the prison transport officers don't care.
The ride for the inmate is uncomfortable as is as they are usually in handcuffs, shackles and are sitting on hard seats.