For me (as my avatar suggests), I've been fascinated with the London Underground - big and full of history. Hundreds of stations, each with personality - many were built in different era by different (often competing) companies, and each company had its own distinct style. Sub-surface stations are not always completely "underground" - many were built with openings in the tunnels to allow smoke to escape during the steam era. Tube stations and lines actually follow the path of streets above because during the time of construction, it was cheaper to bore directly under the streets instead of purchasing the rights to dig under buildings - Many stations are curved, resulting in uneven spacing between the train and platform, leading to the famous "Mind the Gap" warning, and a lot of stations found use as bomb shelters during World War II. Other metro systems may be bigger, newer, cleaner, or more efficient, but London is the one that captured my fascination.
Many stations on the Northern line were built in this style
Some station buildings had other buildings built on top. This is a similar design, but I believe it might have been built by a different company.
Many of the older "tube" stations each had their own unique tile pattern, most likely intended to make it easier for commuters to recognize the different stations. (By the way, someone needs to turn that access panel around on the platform so that the tiles match up)
Until a few years ago (before being taken over by the London Overground), a LU line ran through the Thames Tunnel, one of the first tunnels under a navigable river (opened in 1843). It was first built as a pedestrian tunnel, but was later converted to railway use.
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"Tongue tied and twisted, just an earth-bound misfit, I" - Pink Floyd
(and then the tower cleared me for take off)