The Pennsylvania Railroad (my favorite american railroad) had many giants of steam, 6 examples include the:
Pennsylvania Railroad S1 6-4-4-6 Pennsylvania
Pennsylvania Railroad T1 4-4-4-4 Duplex
Pennsylvania Railroad J1 2-10-4 Texas
Pennsylvania Railroad Q1 4-6-4-4
Pennsylvania Railroad Q2 4-4-6-4 Duplex
And of course the Pennsylvania Railroad S2 6-8-6 Steam Turbine
A PRR T1 will be returning in a few years time, as for the others, they won't be coming back for now.
Also for those that want to know if any of these are available in Trainz:
The PRR S1 6-4-4-6 is available from the Trainz Download Station.
The PRR T1 4-4-4-4 is available as payware from Simulator Central.
And the PRR J1 2-10-4 is available as payware from K&L Trainz.
Also there is a project on the PRR Q2 4-4-6-4 underway at K&L Trainz.
The other two haven't been made for Trainz yet.