although most rapid transit systems are mostly built in underground tunnels, there are some lines which have been built above the street. these "elevated" lines are cheaper to build than subways since no tunneling is required. a well-known mostly elevated rapid transit system is the chicago 'l' serving chicago since 1892. the new york city subway also has many elevated sections similar to chicago. the bay area rapid transit system in the san francisco bay area and the washington metro in the washington, d.c. metropolitan area, two of the newer rapid transit systems built in the us, began running in 1972 and 1976, in that order. although these last two systems have long sections of track without stations and run mostly above ground, in some places in the median strip of highways; all lines travel through subways in the inner city parts.