"Levitating steam tumbrel" seems a good name for it, don't you think?
Prisoner transport in Mechanicsburg is the responsibility of Wilhelm Diamant, although he does not necessarily drive the steam tumbrel himself. He is, however, a familiar face to the regular prisoner-intake guard ✣ , and therefore must accompany new prisoners often enough.
The tumbrel itself ✣ , motivational method aside, is a sturdily-built wood-and-iron wagon, with an armored door at the rear, and only a small barred window under the roof immediately behind the driver. It is clearly labeled "Prisoner Transport" with the badge of that office emblazoned boldly on the sides; it has a ponderous chain-and-trilobite motif on the front, indicating it's Mechanicsburg property even if it's operating under the Baron's name.
No mention is made of any regular intake schedule, but the guard on duty must sign to acknowledge receipt of the prisoner; for efficiency's sake, the "new meat" is usually shackled to a shipment of supplies - food and other necessities - for the inmates in general. The guard is responsible for announcing to the Castle that a new inmate has arrived.