47 were built, and no longer remain in revenue service. 13 are preserved, while 1 has never been retired and is currently part of Union Pacific's "Heritage Fleet". The rest were scrapped.
No North American locomotive has ever been able to match or surpass the DDA40X's horsepower. However, the GE AC6000CW and the EMD SD90MAC have come close, capable of producing 6,000 horsepower.
The EMD DDA40X was mainly built to haul long and heavy trains over the Sherman Hill on UP's Transcontinental route.
They were very reliable and successful like the old UP 4000 Class, or "Big Boy", steam locomotives; whom they're close to the same size (excluding the tender) and have similar strength (yet they unfortunately were the primary replacements). They were much more successful than ALCO's C-855 diesel locomotive set, GE's U50 and U50C, and GE's GTEL Gas Turbine diesel locomotives. They ran more smoothly, had very few flaws, and were much more safer, efficient, and reliable compared to earlier DD diesel locomotives.
The DDA40X is also a record breaking locomotive, being one of the largest diesel locomotives ever built, and being one of the only eight-axle diesel locomotives to operate in North America, and one of the only eight-axle locomotives to be built by EMD of GM.
They were eventually retired by 1986 due to being expensive to maintain and operate, as well as becoming outdated due to not having microprocessor controls like most modern day diesel locomotives (such as the Dash 8 and SD60). Only one (UP #6936) is currently in service and has never been fully retired like UP 844.
The DDA40X is often referred to as the "Centennial" to celebrate the 100th anniversary of the completion of the Transcontinental Railroad.
The DDA40X was one of the first types of EMD locomotives to have a "wide nose". (Before the official "safety-cab" era began in the 1990s.)
Several DDA40X locomotives are preserved, but 6936 is the only to still remain in service only to pull special passenger excursions.
The DD35A was the standard-cab, 5000 horsepower pre cursor of the DDA40X."DD" indicates the unit has 2 trucks, each with 4 powered axles, yet the EMD "DD" units are sometimes referred to by railfans as "Double Diesel".
Large sandboxes were added to the sides of the DD35 and DDA40X above the rear trucks due to a flaw in the DD35's rear sand box which developed cracks that fouled the electrical cabinets with microscopic sand particles.
The 4 axle trucks also required more sand than tradtional 3 axle trucks. Hence, the need for having large sandboxes mounted on the sides of the units.
The first of the engines; 6900, is curently residing at Kenefick Park, Omaha, Nebraska, the current location of 4023 Big Boy, and the town of Union Pacific Headquarters.
Ironically, the UP's renumbered fleet of GE AC6000CW units retain the same numbers that their giant DDA40X predecessors originally retained (such as #6904; a DDA40X which currently resides at the Circus World museum in Wisconsin).