All the advice given here to validate your tickets and by our lodging folks was certainly good info.
We were riding the U3 Metro (what a great transportation system) yesterday when a fellow who was dressed as a workman got on at a station and as soon as the doors closes, whipped out a badge, announced who he was and asked to see validated passes!
We were in the back end of the car with at least 2 mothers with children, a couple of local workman and we tourists (4 of us).
We all had valid passes and he got off the next stop.
Gene