A tunnel motor refers to an ElectroMotive Diesel SD40T-2 or SD45T-2 locomotive, models that were specifically designed and built to deal with tunnel cooling issues. A total of 312 SD40T-2s were built for the Denver & Rio Grande Western and the Southern Pacific (including ten owned - for financial reasons - by the SP's SSW subsidiary). The Rio Grande never owned SD45T-2s, but 247 were built for SP and SSW. The engines feature large cooling intakes near walkway level at the back of the unit. This allowed large amounts of cool air to be pulled from low in the tunnel, rather than running superheated air and exhaust gasses from the upper part of the tunnel through the radiator.