The introduction of the two-class IC system on an hourly schedule at the end of May 1979 brought with it a considerable reduction in railroad postal routes, because IC trains were not supposed to have baggage or mail cars with a few exceptions. In particular, the transport of letters with its fast shipment to ensure early delivery could not be maintained adequately with the remaining transport options. The German Federal Postal System thus now requested the ability to transport letters with a separate network of express and postal trains. Initially, the DB flatly refused these wishes of the Postal System, but under pressure from the Transportation and Postal Minister at that time, Kurt Gscheidle, it had to make several concessions to the German Federal Postal System and also put into effect a postal train network. After being introduced by the German Federal Postal System, these trains were to run as "Post InterCity", but in the end, they were run on the DB as Express InterCity (Expr-IC). The German Federal Postal System could still insist that the punctuality of these Express-IC trains should equal that of the IC trains and the former therefore had to be handled like passenger trains.
As a rule, these mail trains ran at night in the core time of 10 PM to 6 AM. The DB provided its own type Dm and Dms baggage cars as express freight through cars in order to use the unloved postal network to accelerate the railroad's own baggage and express freight service. In the final expansion of the Express Intercity network, 15 trains connected important mail distribution centers in the German Federal Republic, whereby not all Express IC trains ran in pairs.