Which London Underground line was extended to Chesham in 1889?

The Metropolitan line was extended to Chesham in 1889.

Chesham became part of the Metropolitan Railway’s expanding north-western network in 1889. The station remains the north-western terminus of the Metropolitan line and is one of the farthest Underground stations from central London.

The Metropolitan line has a distinctive history because it began as the world’s first underground railway in 1863 and later developed into an extensive suburban and main-line-style route. Its trains serve both dense inner London areas and towns in Buckinghamshire.

Chesham is sometimes confused with Amersham, another Metropolitan line terminus and the station at the highest elevation on the Underground network. Chesham is a branch terminus, while Amersham lies on the main route beyond Moor Park. The two stations are separated by a short branch railway and are both outside Greater London.