This supplemented the UP's fleet of powerful gas turbine GTEL locomotives, the most recent batch of which could output 8500 horsepower.

The Southern Pacific Railroad (who served much of the same territory as UP, a rival) took this into account, as business for SP was growing rapidly.

After much research, the Southern Pacific found that using many diesel–electric locomotives with continual high stress on the traction motors wore out the electrical equipment.

Delivered by ship and unloaded at the Port of Houston, Texas, on October 31, 1961, they featured two Maybach V16 1,770 horsepower (1,320 kW) diesel engines and Voith transmissions.

The Denver and Rio Grande Western Railroad also ordered three units, but found them unsuitable in mountain service, and they were sold to the SP in early 1964.

The first order of the ML-4000s are referred to as cab units, given that they have a fully enclosed car body, similar to that of the EMD F-unit.

The Pacific Locomotive Association (the organization that operates the Niles Canyon Railway) came to SP with a request of a diesel–hydraulic–powered railfan passenger excursion, preferably with an ML-4000 cab unit.

The powered trucks were salvaged from some of the scrapped ML 4000's and sold to Plasser & Theurer, where they were installed on that company's self-propelled ballast cleaners.

It was completely rebuilt to house camera equipment and heavy, thick steel was used for collision protection.

The camera car could be put on the lead of any train, but it mostly made special trips with just one locomotive behind it for power.

Estrada de Ferro Vitória a Minas of Brazil ordered four units built to meter gauge in 1966.

They were moved by the Union Pacific Railroad in summer 2008 from Sacramento to their interchange with Niles Canyon Railway at Hearst in Sunol, California.

In 2013 the PLA was able to obtain a set of Krauss Maffei trucks from a retired Plasser and Theurer ballast cleaning machine from France.