Seattle: The Link
Seattle's Link also has what comes down to an optional payment system that in theory may be spot checked. Of course, the one time I saw anyone asked for their ticket, when they didn't have it they were just told they had to wait for the next train, not actually kicked out of the station.
Most interesting to me was the Westlake station downtown. First, it's a huge space with a marble clad mezzanine level, above. Secondly, the 'track' level is actually just a subterranean street. Buses and Link light rail trains roll through from tunnels heading in either direction.





