Electro-Motive Division Tunnel Motor diesels solved a tricky problem: Locomotives operate in all climates, from scorching desert heat to frigid Canadian winters. An ample supply of fresh air is always required for cooling and combustion purposes but, when trains operate through long tunnels or snowsheds at slow speeds, the ability of a locomotive to receive large amounts of air cool enough to provide proper heat dissipation via the radiator system became an issue for several western railroads, most notably Southern Pacific and Denver & Rio Grande Western). In the early 1970s, the SP and EMD experimented with modifying the air flow on twelve SP SD45s, adding shrouds (dubbed "Elephant Ears") to the outside the radiators to draft cooler air from the lower portion of the air in tunnels.
The positive results of these test units led to the design the SD45T-2, the first Tunnel Motor diesel constructed by EMD. This was a modified version of EMD's SD45-2. Since hot air rises, the modified locomotive has low-mounted air intakes along with larger radiators. These diesels featured high-mounted radiators cooled from below by upward-blowing fans.
Southern Pacific (along with subsidiary St. Louis-Southwestern, or Cotton Belt) was the only railroad to purchase the SD45T-2, ordering 247 locomotives between 1972 and 1975. Later, a Tunnel Motor variant of the highly successful SD40-2, the SD40T-2, gained slightly more orders at 312, split between Southern Pacific/Cotton Belt and Rio Grande, constructed from 1974 to 1980.
In 1996 all Tunnel Motors built for SP/SSW and D&RGW ended up via mergers in the locomotive fleet of the Union Pacific. Most of the Tunnel Motor were purged from UP's roster in the early Twenty-First Century, ending up on dozens of other Class Is, short lines, regionals, and lease fleets across North America.
Southern Pacific SD45T-2 6819, built in March 1972 as SP 9193, was donated to the California State Railroad Museum to be preserved. (Sacramento, California – June 23, 2025)
