Given limited clearance such as in tunnels, the overhead wire may be replaced by a rigid overhead rail. An early example was in the tunnels of the Baltimore Belt Line, where a ∏ section bar (fabricated from three strips of iron and mounted on wood) was used, with the brass contact running inside the groove. When the overhead line was raised in the Simplon Tunnel to accommodate taller rolling stock, a rail was used. A rigid overhead rail may also be used in places where tensioning the wires is impractical, for example on moveable bridges.