Phnom Penh: Solution

This is a T puzzle, as clues by the colors and the words superimposed on the rubbings. Each rubbing clues a station a short distance from MIT and the rubbing is taken in or near that station.

Put these together in the order of a possible trip connecting all these stations: Red line from Central Square to Downtown Crossing, orange to Back Bay, and then back on the orange line to North Station (Back Bay is on the commuter rail, but it is on the South Station commuter rail, not connected to North Station). Alternatively, just anagram. This gives RESIDUAL.