This unit and one B unit were equipped with 12-cylinder Cooper-Bessemer FVBL-12 diesel prime movers (these two were informally known as Model UM-16, 1600 hp), while the other A-B units were equipped with 8-cylinder C-B FVBL-8 diesels (informally known as UM-12 for the 1200 hp they produced). The purpose was to test existing designs in road locomotives. Alco had upset GE by not bidding on a couple of locomotive contracts that GE was interested in, so GE set out on its own to build a road diesel, which in the long run proved fatal to Alco. From 1954-1959, the UM-20 (also known as UM-20B) tested on the Erie, painted in Erie colors and numbered #750A-B-C-D. Since GE's design department had a hand in locomotive designs at Alco, there is a similarity between this design and the FA-1/FA-2 Alcos. These locomotives lead to the design and development of the U25B. After the Erie road tests, the locomotives were returned to GE, where all four units were upgraded to 2000 horsepower locomotives with 12-cylinder C-B FVB12LT diesel prime movers, and sold to the UP on October 21, 1959 as #'s 620-620B-621B-621. They were retired by the UP in 1963 and traded to GE on U50's.

The bridge was built in 1903, with the survey work and planning begun by the White River Railway, which was purchased by the Saint Louis Iron Mountain & Southern Railway on March 26, 1903. At the time, this location was known as Lake's Ferry, but the name was changed to Cotter, named after William Cotter, a manager on the Missouri Pacific Railroad. Both the StLIM&S and MP were interests of Jay Gould, and the purpose of the bridge and route was to connect the two properties. The Army Corps of Engineers determined that the White River was navigable, so a turn span was built to allow passage of river traffic, which all but disappeared after the arrival of the railroad. When the bridge was completed, the span was turned just once for testing, and never turned again. It may appear that there is plenty of room for river barges to pass below, but before the dam was built about ten miles upstream, spring floods would raise the river to within a few feet of the bottom of the bridge, flooding the railroad yard and roundhouse just a few hundred yards downstream.

Alco built this locomotive as the second of five demonstrators and assigned it road number 640-2 (DL-640 was Alco's specification for the RS27, the numeral was used as road numbers) in December 1959 (c/n 83556). It was powered by a 16-cylinder Model 251B prime mover coupled to a GT581 main generator, Type MG-8 power plant regulator, driving four GE 752 traction motors, producing 2400 horsepower. It featured an advanced (in 1959) transistorized control system, known as Type E, which utilized a GY27A1 auxiliary generator as the exciter.

The RS27 was an upgrade of the RS11, using the same basic frame, but packing a 16-cylinder diesel in place of the RS11's 12-cylinder diesel, resulting in the stubby appearance of the short hood as the cab was moved forward to make room. Alco built five DL-640/RS27 demonstrators, three of which conducted shakedown tests on the D&H before moving to the NYC for road tests. Although the demonstrators performed well on the NYC's high speed Flexi-Van trains, only one order resulted, and that was for a single unit to the Green Bay & Western. It took another year before the next order, and that was for four of the demonstrators (#'s 640-2 through 640-5), purchased by the UP in September 1961 and numbered 675-678. Alco demonstrator 640-1 was rebuilt to a Century 424 and sold to Pennsylvania Railroad as their 2415. After retirement on the UP in 1971, two of the former demonstrators (675 and 676) moved to Canada and formed the beginning of a lease fleet for MLW. The sales figures were not too impressive for the RS27; a total of 37 units were built in a production run of a little over one year. This was due to a number of factors, not the least was EMD's introduction of the GP30 and GE's new U25B, all available at about the same time. A personal observation, I have no evidence to back this up, but in researching this, I noted that Alco had really upset the UP with their FA1 model (all of UP's FA1's needed warranty repairs, 90 diesel engine blocks were replaced), and the RS27's were likely a peace offering. These four locomotives were orphans on UP's system. I think Alco recognized that they needed UP's business to survive, and probably gave the UP a good deal on these locomotives. After all, they were demonstrators, and had a lot of miles on them before getting UP paint.