The infrastructure of the London Underground includes 11 lines, with 270 stations. There are two types of line on the London Underground: services that run on the sub-surface network that is just below the surface and use larger trains, or the deep-tube lines, that are for the most part self-contained and use smaller trains. Most of the lines emerge onto the surface outside the central area. The oldest train running on the Underground was introduced in 1972. The Underground is electrified using a four-rail system, the DC traction supply being independent of the running rails. Planned improvements include new stations, line extensions and more lines with automatic train operation (ATO).