General
The tunnel is a critical part of the trunk railway line between London (and southern England generally) and South Wales, and carries an intensive passenger train service, and a significant freight traffic.
The signalling arrangements are such that almost the whole length of the tunnel is a single signal section, limiting the headway of successive trains. The steep gradients (1 in 90 and 1 in 100) make the working of heavy freight trains difficult.
There is a continuous drainage culvert between the tracks to lead ground water away to the lowest point of the tunnel, under Sudbrook Pumping Station, where it is pumped to surface. The hazard of ignited petroleum running into the culvert in the event of derailment of a tank wagon means that special arrangements have to be made to prevent occupation of the tunnel by passenger trains while hazardous liquid loads are being worked through. Special evacuation arrangements are in place to enable the escape of passengers and staff in the event of serious accident in the tunnel.
There is restricted personnel access to the tunnel at Sudbrook Pumping Station, where an iron ladder descends in the shaft of the water pumping main; the ventilation air is pumped in at this point also. The GWR original ventilation arrangement was to extract air at Sudbrook, but the aggressive exhaust gases from the then steam train operation led to premature corrosion of the fan mechanism, and when the pumping engines were replaced in the 1960s the draughting was reversed so that atmospheric air is pumped in to the tunnel exhausting at the tunnel mouths.
In the event of non-availability of the tunnel, trains are diverted via Gloucester.
The specially difficult conditions for infrastructure maintenance in the tunnel, as well as the physical condition of the tunnel structure, require a higher than normal degree of maintenance attention; and access and personal safety difficulties mean that significant work tasks can only be done during temporary line closure.
Some 200 trains per day use the tunnel.
Read more about this topic: Severn Tunnel
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