Rail Link. In 2016, Overhead line equipment (OHLE) was installed in the tunnel to allow the passage of electric traction; this work was undertaken as one element of the wider 21st-century modernisation of the Great Western main line. General The Severn Tunnel forms a critical part of the trunk railway line between southern England and South Wales, and carries an intensive passenger train service as well as significant levels of freight traffic. As of 2012, an average of 200 trains per day use the tunnel. The whole length of the tunnel is controlled as a single signal section, which has the consequence of 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 the surface. The hazard of ignited petroleum running into the culvert in the event of derailment of a tank wagon means special arrangements have to be made to prevent occupation of the tunnel by passenger trains while hazardous liquid loads are being worked through. 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 exhaust gases from steam train operation led to premature corrosion of the fan mechanism. When the Cornish pumping engines were replaced in the 1960s, the draughting was reversed so that atmospheric air is pumped into the tunnel exhausting at the tunnel mouths. On average, it has been determined that around per day of fresh (spring) water are typically being pumped from the tunnel; this is normally released directly into the adjacent River Severn. Attempts have also been made to try to determine the sources of the water which feeds the "Great Spring". The especially 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. Access and personal safety difficulties mean that significant