A snow shed, snow bridge or avalanche gallery is a type of rigid snow-supporting structure for avalanche control (avalanche defense) or to maintain passage in areas where snow removal becomes almost impossible. They can be made of steel, prestressed concrete frames, or timber. These structures can be fully enclosed, like an artificial tunnel, or consist of lattice-like elements. They are typically of robust construction considering the environments they must survive in.
Snow protection is particularly important when routes cross avalanche "chutes", which are natural ravines or other formations that direct or concentrate avalanches.
Snow sheds or avalanche galleries are a common sight on railroads in mountain areas, such as the Donner Pass in the United States or many of the Swiss mountain railways, where tracks are covered with miles of shedding. Although unused today, the Central Pacific railroad had a complete rail yard under roof in the pass. They are also found on especially hazardous stretches of roadway as well. The Trans-Canada Highway between Revelstoke and Golden in British Columbia has several snowsheds covering both carriageways to cope with the heavy snow. Snoqualmie Pass previously had a snow shed for the westbound carriageway of I-90 near Keechelus Lake, though it was replaced with a bridge in 2014. The I-90 snow shed was the last remaining snow shed along an Interstate highway when it was removed.(Wikipedia)