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.