Within the next few years, actual diesel locomotives were introduced to power the streamliners. The General Motors Electro Motive Division (EMD) soon emerged as the world's largest manufacturer of diesel locomotives. They acquired the Winton Engine Co., and their earliest diesel locomotives were built with Winton engines. But the Winton engines were soon superseded by the "567" engine. And the overwhelming number of passenger diesel locomotives were built with various models of the 567 engine.
As with the steam locomotives, the earliest diesel locomotives were custom manufactured for the railroads. Many of the early passenger diesel locomotives were built with twin 12 cylinder engines. This became EMD's sleek "E" series of locomotives. And after a few custom E-unit models, EMD subsequently produced standard locomotive models, available to all railroads. Although certain custom options were available for the standard models.
Over the years, newer versions of the 567 engine models were developed, and newer E-unit models were introduced. The E-units were the most successful diesel locomotives built for the private railroad passenger trains. EMD's largest competitor was Alco (American Locomotive Co.), which produced the "PA" line of passenger locomotives. And Baldwin and Fairbanks Morse were smaller manufacturers of locomotives. But basically, only the E-units lasted into the Amtrak era.
For several western railroads with mountainous terrains, E-units were found to be unsuitable for many of their grades. Those railroads instead opted for EMD F-units for their passenger locomotives. The F-units were streamlined locomotives originally intended for freight service. Each F-unit had a single 16 cylinder engine, and almost always operated in multiple. In addition many railroads used road switchers, often for local passenger trains. EMD's GP7 and GP9 models were the most popular road switchers, while Alco had the "RS" models. As with the F-units, road switchers were mostly freight locomotives. And when a freight model was adapted for passenger service, a steam generator was needed, for the passenger car heating at the time. And for higher passenger train speeds, different gear ratios were used between the traction motors and the axles.
In the late 1960's, a few railroads acquired new six axle passenger locomotives, which were mostly adaptations of high horsepower freight locomotives. Most notably the "SDP" locomotives, which were variations of EMD'S "SD" road switcher freight locomotives. A few railroads also acquired the FP45, which was a variation of the SDP45, except with a full width body which was somewhat more streamlined. Santa Fe also acquired some passenger variations of General Electric's "U" series locomotives. With the formation of Amtrak, the private railroads generally converted these locomotives for freight service. Making only the older locomotives available for Amtrak trains, mostly E-units.