While building the second Cascade Tunnel, the Great Northern ordered five pairs of semi-permanently coupled electric locomotives from Baldwin and Westinghouse. While originally only the first tunnel was electrified at 6.6 kV three-phase AC and operated by ALCO-GE boxcabs, now the complete section between Skykomish and Cascade of 73 miles or 117 km was electrified at 11 kV single-phase AC.

Each locomotive had four powered axles and one leading and trailing axle on each end. Two cabs were provided, but only one had controls and instruments in it and the second could be equipped if needed. Multiple controls were used to operate both locomotives of each pair, but also to combine two pairs if necessary. A Pantograph busbar was provided to electrically connect both locomotives in a pair. Thanks to a motor-generator that was used as rectifier, they could also use regenerative braking.

They could easily accelerate traffic with their power of 1,830 hp, but the Y-1 delivered by ALCO-GE at the same time was heavier and much more powerful at 3,000 hp. Both classes were used together until electric service was stopped on the Cascade line. In 1956 diesels started operation through the Cascade tunnel, aided by portal doors and powerful fans to get the fumes out.