The Channel Tunnel is the longest undersea tunnel in the world. The section under the sea is 38km long. The three tunnels, each 50km long, were bored at an average 40m below the sea bed, and link Folkestone in Kent to Coquelles in Pas-de-Calais.

Eurotunnel shuttles, Eurostar and national freight trains run in the two single track and single direction tunnels. These are connected to a central service tunnel by cross-passages situated every 375m. The service tunnel allows access to maintenance and emergency rescue teams and serves as a safe haven if passengers need to be evacuated in an incident. The service tunnel is a road tunnel used by electric and diesel-powered vehicles. Air pressure is higher in the service tunnel to prevent the ingress of smoke in case of a fire in one of the rail tunnels.

The two rail tunnels are 7.6m in diameter and 30m apart. Each rail tunnel has a single track, overhead line equipment (catenary) and two walkways (one for maintenance purposes and the other for use in the event of an emergency evacuation and on the side nearest the service tunnel). The walkways are also designed to maintain a shuttle upright and in a straight line of travel in the unlikely event of a derailment.

The service tunnel is 4.8m in diameter and lies between the two rail tunnels 15m away from each of them. In normal operations shuttles use the south tunnel in the France – UK direction, and the north tunnel when travelling from the UK to France .

Two undersea crossovers bring flexibility of operation as trains can pass from one tunnel to the other during night maintenance periods to isolate a section of tunnel.

The track in each rail tunnel has two continuously welded rails laid on pre-cast concrete supports embedded in the concrete track bed.

Fixed equipment in the tunnels comes under four categories: electricity and catenary, rail track and signaling, mechanical systems and control and communications.

Cooling pipes, fire mains, signalling equipment and cables are fixed to the sides of the tunnels and are fed by cooling plants at Samphire Hoe in the UK and Sangatte in France .

The overhead catenary supplies traction power to the shuttles as well as to other trains using the Tunnel, e.g. Eurostar and international rail freight trains. The catenary is divided into sections, so that maintenance work can be carried out in stages. Electrical power supplying the tunnels, drainage pumps, lighting and the trains, is provided by substations on each side of the Channel.  In the event of loss of power from one side, the entire system can be supplied from the other side. 
 

The fixed lighting installations can be switched on from the control centre or manually from within the tunnels.  Various fire-protection and detection systems are installed at points along the length of the tunnels.

The entire Eurotunnel transport system is controlled from the RCC (Rail Control Centre). There are two centres, one on each terminal, and each can take turns to take over control of the system. The RCC manages all rail traffic (trains and shuttles) in the tunnels and on the terminals.

The system is in two parts, the Rail Traffic Management (RTM), which controls the rail traffic system, and the Engineering Management System (EMS) which controls the fixed equipment such as ventilation, lighting, power for the catenary, etc.

Although the transport system is automated, controllers are in attendance 24 hours a day, ready to take manual control in the event of technical failure.

The signalling system in the Channel Tunnel is known as TVM 430: it functions by means of data transmission from track to train and is almost identical to the system used on the high-speed TGV Nord-Europe.  Instructions and data are transmitted along the track and then to the locomotive driver by indicator lights in the cab.
All Eurotunnel trains are fitted with vigilance devices and full automatic train protection which minimises the risk of collision in the event of a human error.

After travelling through the tunnel, the through-trains operated by the railway companies then continue their journey on the UK or French rail networks, which are connected to the tunnel tracks at Dollands Moor and Frethun, respectively.

The shuttles operated by Eurotunnel remain within the Eurotunnel system: they travel on a rail loop between the Folkestone and Coquelles terminals, using the south tunnel when going from France to the UK and the north tunnel when going from the UK to France .

A vehicle was specifically designed for travel in the service tunnel. It is multi-functional and is used for maintenance operations and in case of incidents, with the aim of reaching the scene of an incident in minimum time.