X-12 was the name of a US nuclear – powered locomotive project in the 1950s.The X-12, which would have resembled diesel locomotives of the time , was developed in the early 1950s by a research group at the University of Utah with the participation of the five railroad companies Southern Pacific , Union Pacific , Western Pacific , Denver & Rio Grande Western and New York Central.
The locomotive would have contained the driver’s cab , the nuclear reactor , the steam turbine and the four Basset -type direct current generators ; The cooling unit for the reactor water cooling would have been housed in the tender. The nuclear reactor should have a circumference of approximately 90×90×30 cm and be made of stainless steel. It should be surrounded by a jacket filled with water and crossed by 10,000 pencil-thin tubes containing the same water.
The fuel planned was 242 liters of an aqueous uranyl sulfate solution approx. 8 kg of uranium-235. This solution should reach a temperature of 230°C during operation, with boiling prevented by a prevailing pressure of 16 atm . The water contained would have been subjected to constant splitting into hydrogen and oxygen due to the radiation , 10% in thirteen minutes. A resulting water shortage within the solution should be counteracted by passing both gases into a recombination chamber and recombining them back into water using a catalyst .
The water, in turn, which should reach the steam turbine in the form of water vapor through the jacket and the tubes and be condensed there again using a condenser , would be cooled by another water system running through the cooling unit before it is returned to the jacket and the tubes would. The vehicle’s average power output was to be 7,200 hp,  with the possibility of a short-term increase to up to 12,000 hp. A 5,000 t train should be able to accelerate from 0 to 100 km/h in 3:32 minutes.