The Station Keys are key items that appear in Paper Mario: The Thousand-Year Door.
There are two types of Station Key. One has three holes on it while the other only has one. However, they are both green in color.
One of the Station Keys specifically unlocks the entrance to Riverside Station. It is given to Mario by the Excess Express Conductor after the bridge being raised stops their progress. The other Station Keys can be found inside Riverside Station which can unlock doors.