The game is Digimon Super Rumble. A Korean, turn-based MMO that recently got an official English patch.

I was doing a side-quest that led me to a semi-secret area, and when I looked around, I saw these "subway" maps. At first, I didn't understand what I was looking at, then I saw the stations at the top right, and it all clicked into place.

I guess I ain't the sharpest tool in the shed.