Since the railway was the fastest means of transport for a long time, the post also used it to transport of post items. Railway post wagons are classed as passenger coaches and must therefore comply with all the technical regulations for passenger coaches that have been laid down at international level. In railway post wagons, mail was sorted during the journey. Therefore, mail wagons have three types of workrooms:
– The “mail room” with work tables and mail distribution racks
– The “sacking room” with a large work table for sacking out the mail bags
– The “parcel or packing room” which was used to hold as many parcels, etc. as possible
A special external feature were the letter slots in the side walls, into which travellers on the train could drop their mail.

The railway post wagon Post mr-a was an express train post wagon of the German Federal Post Office. The sub-generic signs “a” and “b” indicate special interior fittings of the wagons.