. This meant that the associated locomotive shed at Severn Tunnel Junction (86E), had a large number of pilot and banking locomotives to assist heavy trains through the tunnel. Under typical operations, pilot locomotives usually worked eastwards and were detached at Pilning, and would then work westwards piloting a second train back to the marshalling yard. During the latter days of steam under British Rail, these locomotives were mainly a group of latter-built GWR 5101 Class 2-6-2T locomotives, the bulk of which now form the core preserved stock of that class today. A number of fixed Cornish engines, powered by Lancashire boilers, were used to permanently pump out the Great Spring and other sources of water from the tunnel. These were still in regular use until the 1960s, at which point they were replaced by electrically powered pumps. These pumps and their control systems have since been replaced during the 1990s by privately owned railway infrastructure company Railtrack. During the 1930s, the availability of the reliable fresh water supply from the Great Spring was a significant contributing factor in favour of the selection of an adjacent site to be established as the Royal Navy Propellant Factory, Caerwent. Water was also supplied for paper manufacture to a mill at Sudbrook; this facility has since been closed. On 7 December 1991, the Severn Tunnel rail accident occurred, involving an InterCity 125 that was struck from behind by a Class 155. The subsequent accident investigation, while unable to reach a firm conclusion on the cause, indicated that the axle counters used for detecting train movements in the tunnel may have been accidentally reset. The Second Severn Crossing, which was built during the 1990s, crosses over the tunnel via a "ground level bridge" on the English side, near the Salmon Pool. This bridge is supported in such a way that no load is imposed on the tunnel. During that bridge's construction, the opportunity was taken to renew the concrete cap above the tunnel in the Salmon Pool. In 2002, two Class 121s were overhauled by LNWR, Crewe for use as a Network Rail emergency train that was stabled near Severn Tunnel Junction station. They were removed in 2008 having never been used. Car transport During 1924, the Great Western Railway started a car shuttle train service using the tunnel, which would transport cars on rail trucks through the tunnel between Pilning and Severn Tunnel