Waaaay back ca 1915, PRR electrified it's heaviest commute line from Paoli to Broad St , Phila. with AC MUs, or as now said EMUs.

Almost all PRR trains entered and left Broad St., a stub end terminal, and all steam trains had to be switched to get the proper order, or at minimum the locomotive had to come out and be turned. Some [many, almost all?] tracks required that the entire train be pulled out if the locomotive could not escape on the adjacent track .One of the BIG savings was the ability to simply change ends. Capacity of the terminal was increased due to time savings in clearing a track.
Electric train length could be varied by cutting off cars.

In the present era push/pull nullifies the first triumph at Broad St.
The Metroliners were emu for similar reasons, and are now gone in favor of locomotive hauled trainsets
The British appear to have refined the DMU and use them extensively. That fact surprises me, as their development of diesel locomotives was almost pathetic, with a very few exceptions, the Deltic being case in point

Thanks BOB2