After World War I, the Missouri Pacific ordered a 4-8-2 that would be faster than the MT-63, but had to be limited to an adhesive weight of 226,000 pounds. The goal was to use the heating surface as best as possible, what was accomplished by curved circulating plates and thermic syphons. They also got a large superheater with a surface of more than 1,000 square feet. With a driver diameter of 73 inches, it was designated MT-73. Numbers 5308 to 5312 were built by ALCO-Schenectady in 1921, followed by 5313 to 5316 in 1923.

Since these were still not powerful enough for some passenger services, the following two batches were heavier, had a larger boiler, a pressure of 250 instead of 210 psi and roller bearings. These were 5335 to 5339 built by ALCO-Brooks in 1927 and 5340 to 5344 built by ALCO-Schenectady in 1930. In the result these were considerably more powerful, but could only be used on lines which had been upgraded for the higher weight. While the first two batches were retired in 1955, the last two already followed in the next year.