Although the conversion from steam to other types of traction in the United States had begun before World War II, there were still no single diesel locomotives to replace the large steam locomotives. Since the diesel engines at that time did not yet produce 2,000 hp and there were steam locomotives with more than 5,000 hp to be replaced, experiments were carried out with other energy sources. There were also other attempts with gas turbines, in which the power was transmitted electrically, just like with the diesel locomotives. However, the Union Pacific was the only railroad company that purchased entire series of gas turbine-electric locomotives and used them profitably on a large scale. The turbines, generators and traction motors came from General Electric, while ALCO manufactured the mechanical part.
A key consideration when using gas turbines was the high fuel consumption, which was about twice as high compared to diesel-electric locomotives. The solution was to use heavy fuel oil, which was a waste product from refineries in large quantities and therefore cheap to obtain. However, there were problems with the very high viscosity and the dirt particles it contained, which could be avoided by heating the oil to around 200 degrees Fahrenheit and filtering it accordingly.
The 1948 prototype stood on four two-axle bogies and had a 4,800 hp turbine, of which 4,500 hp was available for traction. The engine had two driver's cabs, which on the outside had a streamlined shape similar to that of ALCO's FA locomotives. For starting the turbine, a 250 hp Cummins diesel was used, which itself had a generator and used the electricity generated to turn the generator connected to the turbine, thus causing the turbine to rotate. The turbine was initially powered by diesel until a steam generator had heated the heavy oil to the required temperature. The turbine ran constantly at nominal speed during operation, while a change in power setting only resulted in a change in the turbine and generator load.
Second generation No. 64 together with FEF-3 No. 826 in front of a fruit train
H.R. Griffiths Jr.
The first generation of ten production engines were built in 1952 and 1953 and were largely based on the prototype. One difference that was visible from afar, however, was that there was only one driver's cab. After the side air intakes were replaced with an intake on the roof in one example, the last four examples were delivered in this form. The engines hauled large numbers of freight trains across the Rocky Mountains and were soon given a 24,000-gallon tender in addition to their 7,200-gallon tank. Propane firing was tested on one example in order to reduce fouling of the turbine blades. This was just as unsuccessful as coupling two locomotives together to form a double unit with 9,000 hp. In the latter case, the exhaust gases from the front turbine often caused flameouts in the rear turbine in tunnels.
The 15 locomotives of the second generation were delivered in 1954 and were technically still largely the same as the first generation. The hoods were now narrower and allowed running boards on both sides of the locomotives, which earned them the nickname “Veranda”. Since the previous attempts with two turbine locomotives coupled to each other were unsuccessful, these locomotives were provided with multiple controls in order to be able to control additional diesel locomotives.
With the introduction of a new turbine with 8,500 hp, the third generation was created, which was delivered 30 times between 1958 and 1961 and replaced the locomotives of the first and second generation. Now a locomotive consisted of two six-axle parts. In the front part, besides the driver's cab, there was the now 850 hp auxiliary diesel and other machinery, while the second part housed the turbine and its generator. Now the 24,000 gallon tender was used from the start and it was now insulated to allow it to be loaded with preheated and filtered oil to avoid warming up on the loco.
Side view of the third generation
Larryzap 
The turbine was designed to achieve rated power at 6,000 feet and in very hot temperatures so as not to suffer performance losses. Attempts were made to use the higher turbine output possible at sea level and at lower temperatures by also equipping the tender with traction motors. Since this effort was not worthwhile, the project was not pursued further.
The locomotives were used so intensively that the Union Pacific stated at times that ten percent of its freight traffic was handled by turbine locomotives. On routes with a ruling grade of 1.14 percent, the permissible train weight was set at 5,180 short tons. The locomotives were phased out in 1969, when the price of heavy oil had risen sharply due to increased uses. The practice of having heavy trains pulled by several diesel locomotives in multiple was then continued. By then all locomotives had clocked up more than a million miles. Today there are only two pieces of the third generation remaining that are on static display.