"40ft Boxcar USRA SS SLSF" from Trainz Forge - New Releases - Item 16 "USRA Single Sheath Boxcars" scroll down to SLSF
Ah, you young guys with the (more or less) up-to-date stuff. One of these days, I will break down and upgrade, too, but I have done so much in TS12 I fear losing it.