signal gantry

A signal gantry is a bridge-like steel or lattice structure spanning one or more railway tracks, built to mount signal heads and other apparatus above the running lines. Unlike a signal post, which stands beside the track on a single support, a gantry spans across the tracks and typically carries multiple signal aspects, allowing a driver to read all relevant signals in a single glance without parallax error. The structure is anchored at both ends, or at a single central support for shorter installations over two or three tracks.

Gantries become necessary where space alongside the track is unavailable, where many signals must be grouped together, or where the infrastructure layout requires signals to be positioned well clear of the track boundary. A gantry carrying signals for multiple lines or directions reduces sightline clutter and allows signals to be positioned at a consistent height and angle relative to the driver's line of sight. The platform or arms of the gantry hold the signal heads, which are usually bolted in place with provision for rotation to alter their angle.

Materials and Structure

Steel gantries are most common in modern railways. The framework consists of hollow rectangular or tubular sections, welded or bolted together, with a top boom or series of cantilever arms extending outward to position signal heads. The entire assembly must be rated for wind loading, the weight of the signals themselves, and any additional equipment such as junction indicators or repeater lights. Support columns are often tapered toward the top to reduce wind resistance. Gantries are usually painted grey or left unpainted steel, with the signal heads mounted in standardised positions.

Maintenance of a gantry involves regular inspection of welds, bolted connections, and any rust or fatigue cracks, especially in the welded zones around the arms. The electrical cabling feeding the signals runs through conduit either within the support columns or along the underside of the boom. Some gantries include provision for staff access via an internal ladder or external platform, though this is uncommon in modern installations due to electrification and working-line safety concerns.

In older railways, particularly on multi-track trunk routes, gantries were constructed from wrought iron or cast iron and remain in use. Modern replacements are invariably steel. A gantry designed for four tracks may span up to 20 metres, while a two-track gantry typically spans 8 to 12 metres. The height of the signal heads is set according to the sightline requirements of the line speed and track geometry, usually around 4 to 6 metres above rail level.