Others have described Underground as Ticket to Ride on the subway, and there's a fair bit of truth to that. In general, if you like Ticket to Ride, you'll like Underground. It's not as simple as connecting two end points as in TtR, but involves worrying about loops, rail terminals, and more. It's not difficult to learn, but there are a few fiddly bits to keep sorted in the box and board. For those of us who know London well, there's a familiarity to the game, but those who've never been exposed to London won't have the same feeling.
As a good diversion for an hour or less, Underground works well. Easy to learn, interesting strategic choices, and a nice deviation from games like Ticket to Ride!