EMD SW1504

The EMD SW1504 is a diesel-electric switching locomotive produced by General Motors' Electro-Motive Division and sold exclusively to Ferrocarriles Nacionales de México, the national railway system of Mexico. All 60 units were constructed during a concentrated production window between May and August of 1973, making the SW1504 a relatively specialized and limited-production model within the broader EMD switcher family. The designation reflects the locomotive's 1,500 horsepower output combined with its role as a four-axle switcher, and its sale to a single customer gave the type a distinctly regional character uncommon among EMD's switcher offerings of that era. As Ferrocarriles Nacionales de México underwent restructuring and eventual dissolution in the late 1990s, the SW1504 fleet was distributed among the various regional and private railroad operators that emerged from the breakup of the national system. This fragmentation of ownership meant that individual locomotives ended up working under several different railroad names and liveries across Mexico. Some units eventually made their way into the hands of leasing companies based in the United States, further dispersing the class beyond its original operational territory. The SW1504 occupies a modest but notable place in the history of North American railroad motive power as an example of EMD tailoring its switcher product line to meet the specific procurement needs of a foreign national railway. Although produced in relatively small numbers and confined largely to Mexican railroad service for most of their careers, the locomotives demonstrated the durability typical of EMD switcher designs and continued to find employment in yard and industrial switching work well after the dissolution of the railroad system for which they were originally built.

The SW1504 is built on a B-B wheel arrangement, meaning it rides on two two-axle trucks, with all four axles powered. The locomotive produces 1,500 horsepower and employs direct current traction motors, a configuration consistent with EMD's established switcher engineering practices of the early 1970s. Notably, the SW1504 departs from the tradition of EMD-designed prime movers by utilizing a Caterpillar diesel engine rather than one of EMD's own engine families, a somewhat unusual choice that distinguishes the model from the majority of EMD's domestic switcher production during that period. This use of a Caterpillar powerplant may have reflected the specific operational and maintenance preferences of Ferrocarriles Nacionales de México, or considerations related to parts availability and service infrastructure within Mexico at the time of the order. The overall mechanical layout of the SW1504 otherwise follows the general conventions of EMD's switcher line, with the carbody and underframe configuration suited to low-speed yard work and industrial switching assignments. The combination of a proven North American traction system with a Caterpillar engine gave the SW1504 a hybrid character that set it apart from more conventional EMD switcher models of comparable horsepower.

—