So I know that you can walk over the Queensboro Bridge, but can you stop off at Roosevelt Island? I know that there is also a Roosevelt Island Bridge, but I want to know specifically about the Queensboro Bridge.
Nope, not unless you have a time machine. There used to be an elevator, but it doesn't exist anymore. You can walk over the island, though, then take the subway back to the island.
More info: http://rioc.ny.gov/transportation.htm