The Union Pacific Railroad earned a reputation for using very large and
powerful steam locomotives and in 1941 began using the largest of them all,
the Big Boy
, to carry freight over the Rocky
In the diesel era, the tradition continued with the use of the most powerful
locomotives available. In 1969 UP purchased the very largest and most
powerful diesel ever built, the DDA40X, with 6600 HP provided by two diesel
engines (prime movers) mounted on one frame,
These 270 ton, 98 foot long behemoths were built by the Electro-Motive
Division of General Motors Corp. (EMD) and were named "Centennial" in
honor of the 100th anniversary of the "Driving
of the Golden Spike Ceremony" (May 10, 1869). In all, 47 units were
purchased (numbered 6900-6946) beginning in May of 1969 with deliveries
continuing through 1971. They were numbered using the "6900" series to
mark the '69 dates.
The "Centennials" were designed for high-speed freight and by 1980 had
successfully run up an average of 2 million miles a piece. With the decline
of freight movement in 1980 the fleet of DDA40Xs were taken out of service
and put in desert storage at
Yermo, CA. Then in March of 1984 an economic recovery brought a
demand for more motive power and 25 were returned to service. However, high
maintenance costs caused the retirement of most of them by the close of
Today 13 of these double diesel locomotives survive and can be visited as
shown below. One unit #6936 has recently had a major overhaul and is still
owned and regularly operated by the UPRR.
When diesels took over on the Union Pacific, the power of steam was not
easily matched with some trains requiring as many as eight diesel locomotives to
move freight over the continental divide. UP management wanted a better way
of providing motive power and EMD responded with a 5000 HP cabless booster
unit. This unit was designated "DD35" and was built with off-the-shelf
components and was essentially two standard GP35's on a common frame and ran
on two four-axle trucks.
Many considered the four-axle truck arrangement to be very hard on tracks,
but built as a cabless booster unit it would never be in the lead, and the
effect on the tracks would be acceptable. EMD built two demo units for UP
(road numbers 72B & 73B) and
delivered them in September, 1963.
In April, 1964, three demo units were delivered to Southern Pacific (road
numbers 8400-8402 which later became 9500-9502 and still later became 9900-9902).
SP did not respond with purchases of additional units and only used the three
booster units they bought sparingly until they were retired in the 1970's.
Meanwhile, Union Pacific, which had and still has excellent tracks, ordered
25 of the DD35
booster units (road numbers 74B through 98B) (Railfan.net Usenet
ABPR Picture Archives) and requested that a cab be added to all future
orders. UP later bought 15 of the units with cabs which were designated "DD35A"
(road numbers 70 through 84).
The moderate success of the DD35A and the
introduction of the GP40 led very quickly to the development of the DDA40X
which was basically the DD35A with
increased HP. Union Pacific bought 47 of these units. The cost
effectiveness of the "DD's" was never fully disclosed, however they were
shorter than two GP's and obviously saved on the cost of cabs. But the
greatest savings would have been from the removal of a great deal of the
troublesome electrical control equipment that was required to operate
Continued development of more reliable and easier to maintain modular
electrical systems along with the introduction of the 20 cylinder, 3600 HP
"SD45-2" allowed for convenient selection of available HP and reduced the
appeal of double diesel locomotives.
Based on the HP rating the DDA40X is the most powerful diesel locomotive ever
built and coupled with the special introduction and the romance in the name
"Centennial" it has become an icon for diesel rail fans.