shoofly

A shoofly is a temporary section of railroad track laid alongside the main line to divert traffic while the original track is taken out of service for repair, inspection, or replacement. It consists of a lightweight rail bed, typically 300 to 500 feet long, with switch mechanisms at each end to guide rolling stock off the primary route and back on again. The shoofly itself is often laid on wooden sleepers or ballast and can be assembled and dismantled repeatedly at the same location or moved to different sections of line as needed.

Shooflies are essential infrastructure on working railroads because they allow trains to continue operating through a corridor while track gangs conduct major work underneath. Without one, an entire section of line must be closed to traffic, costing operators revenue and disrupting schedules. The bypass is particularly valuable for ballast renewal, tie replacement, rail grinding, or drainage work that requires unfettered access to the roadbed and does not allow trains to pass overhead.

Construction and temporary nature

A typical shoofly is built from 80 to 115 pound rail stock, lighter than the main line specification, and constructed to lower geometric standards. Speed restrictions of 15 to 25 mph are normal. The switch work at entry and exit requires manual or power-operated points to safely transfer weight from one rail to another. Proper ballast, drainage, and alignment are still essential to prevent derailment, but the structure is designed to be cost-effective for single-season or multi-season use rather than permanent installation.

The term's origin is uncertain but may derive from the notion of shooing or directing traffic to the side, much as one might shoo a fly away. Railroads in North America, Britain, and Australasia all use the word in this sense. The shoofly distinguishes itself from a true branch line or siding in that it exists solely to maintain the main line and carries no revenue traffic of its own; once the primary work is complete, it is typically abandoned and the rail recovered for reuse elsewhere.

Maintenance planning requires the shoofly to be constructed before major track work begins, adding to project cost and lead time. Poor drainage, inadequate ballast, or wear from repeated train passages can cause the temporary track to settle, creating speed restrictions that worsen over the work season. Track geometry must be monitored closely, and the shoofly is often one of the first items inspected when train performance degrades during a scheduled maintenance campaign.