The Budapest Metro is the rapid transit system in the Hungarian capital Budapest. It is the world's oldest electrified underground railway system, and the second oldest underground railway system with multiple stations, after the originally steam-powered London Underground. Budapest's iconic Line 1 was completed in 1896.
A railway electrification system supplies electric power to railway trains and trams without an on-board prime mover or local fuel supply. Electric railways use either electric locomotives (hauling passengers or freight in separate cars), electric multiple units (passenger cars with their own motors) or both. Electricity is typically generated in large and relatively efficient generating stations, transmitted to the railway network and distributed to the trains. Some electric railways have their own dedicated generating stations and transmission lines, but most purchase power from an electric utility. The railway usually provides its own distribution lines, switches, and transformers.
Power is supplied to moving trains with a (nearly) continuous conductor running along the track that usually takes one of two forms: an overhead line, suspended from poles or towers along the track or from structure or tunnel ceilings, or a third rail mounted at track level and contacted by a sliding "pickup shoe". Both overhead wire and third-rail systems usually use the running rails as the return conductor, but some systems use a separate fourth rail for this purpose.
The metro consists of four lines (M1–M4), each denoted by a different colour. M1 (yellow) runs from Mexikói út south-west towards the river. The M2 (red) line travels east–west through the city, crossing the Danube. The M3 (blue) runs in a broadly north–south alignment, interchanging with the three other lines. The M4 (green) line commences at Keleti pályaudvar and travels south-west, crossing the river, to terminate at Kelenföld vasútállomás.
According to Wikipedia