A loading gauge is a diagram or physical structure that defines the maximum height and width dimensions in railway vehicles and their loads. Their purpose is to ensure that rail vehicles can pass safely through tunnels and under bridges, and keep clear of platforms, trackside buildings and structures. Classification systems vary between different countries, and gauges may vary across a network, even if the track gauge is uniform.
The clearance space between a train and the tunnel is often small. Pictured is a London Underground Northern line 1995 Stock train emerging from the tunnel north of Hendon Central station.
The London Underground utilises differing loading gauges: a Metropolitan line A Stock sub-surface train (left) passes a Piccadilly line 1973 Stock tube train (right).
Zürich – Lucerne IC 2000 double-decker Intercity train
Double-decker carriage as used on French TGV railways
The Channel Tunnel, sometimes referred to informally as the Chunnel, is a 50.46-kilometre (31.35 mi) undersea railway tunnel, opened in 1994, that connects Folkestone with Coquelles beneath the English Channel at the Strait of Dover. It is the only fixed link between the island of Great Britain and the European mainland. At its lowest point, it is 75 metres (246 ft) below the sea bed and 115 metres (377 ft) below sea level. At 37.9 kilometres (23.5 mi), it has the longest underwater section of any tunnel in the world and is the third-longest railway tunnel in the world. The speed limit for trains through the tunnel is 160 kilometres per hour (99 mph). The tunnel is owned and operated by Getlink, formerly Groupe Eurotunnel.
One of the southern tunnel boring machines
Class 319 EMUs ran excursions into the tunnel from Sandling railway station on 7 May 1994, the first passenger trains to go through the Channel Tunnel.
The Channel Tunnel exhibit at the National Railway Museum in York, England, showing the circular cross section of the tunnel with the overhead line powering a Eurostar train. Also visible is the segmented tunnel lining.