The route these trains operated was the B&O's other main line that reached St. Louis, branching from the Chicago route at the important terminal in Cumberland, Maryland (which remains a notable location today under CSX). It was then off to Grafton, passing through the rolling hills along the Parkersburg Branch to Parkersburg, West Virginia and crossing the Ohio River into the relative flat country of Ohio. This was followed by Cincinnati and the farmland of southern Indiana until the trains finally reached the Gateway To The West. Compared to the Cap's routing the National and Diplomat's was far less populated but remained popular enough for a number of years to warrant the B&O fielding two trains.
During the Diplomat's peak years it ran with heavyweight coaches and diners along with sleepers to New York in the east and Oklahoma City/San Antonio in the west via the St. Louis San Francisco Railway's (Frisco) Meteor and Texas Special (operated jointly with the Missouri-Kansas-Texas Railroad, or Katy). According to Craig Sanders' book Limiteds, Locals, And Expresses In Indiana, 1838-1971 the Diplomat of the 1940s carried three coaches, one diner, and two heavyweight sleepers. Surprisingly, despite its status it was an early candidate for dieselization, reequipped with new Electro-Motive E6As/Bs in 1942. Following the war in 1945 the B&O embarked on a campaign to heavily update its passenger fleet by either acquiring new, rebuilding additional heavyweight cars, or purchasing second-hand equipment from other railroads.
This task was accomplished from the end of the decade through the early 1950s and included more new diesels like the E7s, E8s, and even a few E9s (the Columbian, however, operated with new F3s). While the Diplomat was never streamlined it did receive some of this new power. Unfortunately, for the industry as a whole the return of post-depression passenger traffic during the war was not sustained after the end of World War II. As early as the 1950s the B&O considered combing the Diplomat and National since the former, even by then, was normally running with only two coaches except on weekends. After the 1957 holiday season the B&O stopped providing the stewardess-nurse service aboard the train.
Then on April 27, 1958 the B&O ended all passenger service to New York City, ceding that market to rival Pennsylvania. As such, all long-distance trains like the Diplomat were then truncated to only Baltimore/D.C. while the regional Royal Blue (Baltimore - Philadelphia - New York) was canceled altogether. The 1950s were not good years for the B&O, which was struggling to survive itself and despite its misgivings of cutting back passenger services the railroad simply had no choice in an effort to stave off bankruptcy. It further cutback the Diplomat east of Cincinnati after September 17, 1960 in part due to lack of traffic in that particular market (this was the same corridor the flashy, streamlined Cincinnatian ran but lasted only a few years because of the same problem).
The move enabled the railroad to save $1.2 million annually. During its final years the Diplomat still operated between St. Louis and Cinncy with a coach, diner-lounge, and baggage lounge. Finally, the railroad pulled the plug, canceling the train after April 30, 1961. In an interesting move the B&O brought back the name in 1964 for the former Shenandoah, which served Chicago to Baltimore/D.C. Whatever the railroad's reasoning in doing this it apparently was not happy with the move and returned the Shenandoah name in 1970, just a little more than a year prior to the start of Amtrak. Until the end during the spring of 1971 the B&O's remaining trains continued to offer high-class service, clean accommodations, and a fast, on-time schedule.
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