The DRG introduced on 1 June 1923 the FD-trains into the timetable. The abbreviation FD stands for Fern-Durchgangszug. The FD-trains were besides the international L-trains, which were exploited by CIWL and Mitropa the best what the DRG could offer in long-distance passenger travel. FD-trains can be compared with the present InterCity trains. The FD-train services were mainly long-distance services between the larger German cities. The trains had only first and second class seats and were equiped with a dining car from Mitropa. Some FD-trains who rode in the evenings and during the nights had sleeping cars with sleeping accomodation. The sleeping cars were from Mitropa.
The FD-network started on 1 June 1923 with the routes Berlin-Hamburg, Berlin-Köln and Berlin-München. Soon the network was expanded with other connections who were mainly from the capital Berlin. So arised a star shaped network from Berlin. There were only a few lines who were not from Berlin.
From May 1923 the DRG also put in service new diesel trainsets. These trainsets were the SVT-trainsets. These trains were indicated as FDt trains (t stands for triebwagen). Soon this network was expanded with more connections and new SVT-trainsets. More information about these trainsets can be found here: trainsets epoch II.
The rolling stock what was used for the FD-trains was always the newest rolling-stock from the DRG. In the beginning these coaches were mainly of type "Hechtcoaches" and from 1928 there were coaches of Bauart 1928 too. When from 1938 the skirted coaches were introduced, were these the first to be riding in FD-trains. See also standard design coaches.
It was the same for the locomotives, the first FD-trains were pulled by former states railway locomotives, but when the standard design locomotives were introduced they were used for FD-trains.
The average speed for FD-trains was 100-120 km/h. During the DRG era there were many FD-trains in service on many routes in Germany. A couple of trains were in service on international routes.