Have one end of the subway smashed to shit and map the destruction maybe? Slightly cliche (just a bit) but can be undeniably effective for a cool looking map.
Other alternatives include working subway trains that move on schedule to show off scripting, or a sekrit base (also slightly cliched) hidden via an entrance down a tunnel.
I plan to include working trains next. Other than that there will be more traces of use, e.g. rubber on the floor