If railroads had diseases, the Southern Pacific would have been diagnosed with asthma. The combination of high altitudes and numerous tunnels and snow sheds (tunnel-like sheds that protect track from snowfalls and avalanches) on its mountain divisions caused perennial breathing problems for SP locomotives and crews. In the steam era, this led to the development of cab-forward articulateds that allowed crew members to breathe tunnel air before the locomotive had polluted it. In the diesel era, the solution was the Tunnel Motor.
In a normal diesel locomotive, cooling air for the radiators is taken in near the top of the car body. In a long tunnel, however, hot engine exhaust and heated air from the radiators collects at the top of the tunnel. This hot air is taken into the radiators and cannot cool the engine sufficiently. In a multiple-unit diesel lashup, the result is that trailing units may overheat and automatically shut down. In a worst-case situation, the extra load will cause the overworked lead units to fail and leave the crew stranded in the tunnel. Electro-Motive's answer for the Southern Pacific was a modification of its SD45, with the air intakes moved lower on the engine so they could take in cooler air. Extra-large radiators were placed on the roof of the engine, with the fans located below them to blow cooler air up from below.
The first SD45T-2 Tunnel Motors were delivered in February 1972, and were among the first "Dash-2" versions of EMD's muscular 20-cylinder, 3600 hp freighter. Responding to complaints of crankshaft and bearing failures on earlier SD45s, EMD had beefed up its model 645 diesel motor. Perhaps more important, the Dash-2 series' introduction of solid-state electronics ushered in the third generation of diesel technology. Transistors and circuit boards replaced the massive switches, contacts, interlocks, and relays of earlier diesels, making possible a range of innovations that would make life easier for diesel engineers in coming years.