from the colonial period, such as the now-defunct trams and the KTM rail lines, used the metre gauge of 1,000mm. However several military railways in former British military bases also used the standard gauge of 1,435mm. The contemporary MRT system uses the 1,435mm standard track gauge with the elevated lines also having guard rails to prevent a derailed vehicle from striking fixed obstructions and viaduct parapets by keeping derailed wheels adjacent to the running rails. Three types of rail tracks are used on the MRT: ballasted track, fixed slab track and floating slab track; floating slab tracks are used at sections of the rail network located beneath densely populated and built-up areas to alleviate vibration and noise transmission to neighbouring buildings, by means of interposing rubberised supports between the tunnel wall and the tracks. When initially built, the MRT used swingnose crossings for 26 out of 131 railway points to minimise noise. From July to November 2000, these swingnose crossings were replaced with conventional ones for reliability reasons. The structure gauge of the MRT is based upon the kinematic envelope such that each point on the perimeter of the kinematic envelope is enlarged vertically upwards by 50mm and horizontally by 100mm. The Sengkang-Punggol LRT and Changi Airport Skytrain use a 1,880mm broad gauge guideway while the Bukit Panjang LRT line relies on a central guide rail. Coupling KTM uses AAR couplers on its trains. All trains on the MRT network, including work trains, use Scharfenberg type 35 couplers. The Bukit Panjang LRT uses BSI (Bergische Stahl Industrie) couplers whereas the Mitsubishi Crystal Mover vehicles used on the Sengkang and Punggol LRT use the compact tight coupler from Japan Steel Works. Railway electrification The entire MRT network uses the 750V DC third rail with the exception of the North East Line, which uses 1,500V overhead catenary. The Bukit Panjang LRT uses a 650V three phase AC third rail located between the guide beams whereas the Sengkang and Punggol LRT use a side-mounted 750V DC third rail. Railway signalling The KTM was known to use token signalling in Singapore as the main line between Woodlands and Tanjong Pagar was a single track line. In contrast the MRT system uses contemporary technology for its signalling systems such as computer-based interlocking, automatic train protection, automatic train operation and automatic train supervision. Although the original signalling system used on the North South and East West