"get off the underground" is correct and usable in written English.
You can use it in a sentence like: "I had to get off the underground at Oxford Circus to change trains."
I got off the underground at Paddington and started walking – and talking to the people on the boats along the way (literally along the way, since they're going at walking speed).
One day, a commuter getting off the subway along William Street will be able to walk underground as far as the World Financial Center.
Several times I had to get off the train and walk miles of blocks to my destination; underground I felt overwhelmed by crowds, claustrophobic.
Get off, get off the rotten world!
Carefully get off the ball.
It also works offline, meaning even being underground on the Tube is no barrier to knowing where it is possible to get off easily.
A central aspect of everyday symbol use is the use of such symbols to guide effective action: we can use a map of an underground rail system to make good decisions about where to get off and on.
This has potential health benefits, in that players gain extra points for getting off the tube a stop early or for using "Boris" bikes rather than the underground.
