Ed, You are quite right on this one. Steam is WAY behind the times. The last advertised add-on was June 27, 2011, and it's for the Duchess. Since then, quiet as a dead cricket. They do not have the Blue Comet or the Aerotrain at all, and there is no sign of them having them in the forseeable future.
Sena, I used to have the Steam version and realized it was way inferior to the straight up version. My advice would be to at least consider purchasing a NON-Steam version. Only then will you get to enjoy the Blue Comet. DEFINATELY hold onto the add on pack somewhere safe. If you do buy a straight version, the add on will still be accepted by it.