The Flyer sets are in S gauge, hence the Flyer, as it is what became of the old American Flyer. Theoretically, the scale should be 1/64; this would be disappointing for most gamers as they are used to having everything too big for no real reason, though when you say 28mm it would be mathematically correct. I could not guarantee that the scale is correct, as these are built to follow older, more toylike patterns.
It would be quite happy on the games table, not as silly as HO, but not as big as O, so long as someone doesn't try stupid things like trying to put gaming figures right into the train...
The Ready-to-Play series are not the same. The Lionel site implies that they are comparable to G, but have an exclusive track system which does not work with G track, and all available information suggests that the train will be vastly too big. Unless of course you often game in car modelling scales.