the Lea Valley Lines. The weekday services on the mainline are as follows: Infrastructure The line was initially gauge, but between 5 September and 7 October 1844 it was converted to . Currently, the line has double track for most of its length, with two exceptions at Stansted Airport Tunnel and at . There is also a short section of quadruple track between and , from which the West Anglia Main Line runs alongside the Great Eastern Main Line to as two of six tracks into the termini. The line is electrified at 25 kV AC and has a loading gauge of W8 except for the Stansted branch, which is W6. Line-side train monitoring equipment includes hot axle box detectors (HABD) on the up main south of Newport (39 miles 48 chains from Liverpool Street) and on the down main north of Shepreth Branch Junction (53 miles 10 chains). There are no wheel impact load detectors (WILD) ‘Wheelchex’ on the line. Tunnels and viaducts Major civil engineering structures on the West Anglia Main Line include the following. Locomotives and rolling stock Throughout the steam era trains were predominantly hauled by Great Eastern Railway (or its constituent companies') locomotives: when steam ended in East Anglia in the 1960s some of these locomotives were still operated – see Stratford TMD and Great Eastern Railway. After the grouping of 1923 LNER-designed locomotives were used with the B17 4-6-0 class working many main line services. Following nationalisation in 1948 British Railways introduced the Britannia 4-6-2 class on some main line services until succeeded by diesels in the late 1950s. East Anglia was the first area to be worked completely by diesel trains with Class 31s taking over some express workings. These were succeeded by more powerful Class 37 and Class 47 until full electrification to King's Lynn in the 1980s when Class 86 locomotives took over. Suburban services from about 1958 were operated by Class 125 DMUs, and following the 1969 electrification Class 305 and Class 308 units. Other units from the GE section such as Class 302 and 306 also operated services during this period, with Class 310s temporarily operating once electrification to Cambridge was complete. These first-generation units were replaced soon after by Class 315 and Class 317 units. In 2011, new Bombardier Class 379s began operation mainly to replace class 317 from Stansted Express duties to mainline services (which in-turn relocated