T-1, T-100, QT (Quenched and Tempered) 100 ... Same - Same just different terminology depending on what your local steel shop calls it.

Main rails should be QT-100.

Cross members will be "Mild" Steel, and should be "Wide Flange" available at just about any steel supplier.

I pre-heat rails, then weld out with Hobart Fabco110 (Dual Shield Flux Core) or with 110-18 to match the characteristics of the stronger of the 2 metals, which is the QT-100.

Other than the way the damage was caused, it is a pretty common, and straight forward repair.

Have a Good Night.