Before there were double stacks there was TOFC. Trailer On Flat Car revolutionized railroading in the 60's. It wasn't as efficient as it could have been but it removed the time delay of loading and transloading and unloading the 40 foot box car. It was sad to see the demise of the billboard sided box but it saved railroads then just like the double stack has today... reducing the overhead cost per mile incredibly (less non-revenue weight) and the burden of lining up the cars so the trailers could be driven on or off.
Here a Santa Fe GP20 leads a duo of U-boat's west on the transcon past Joliet Station where mail awaits the arrival of a passenger train behind. This predates the Super-C but TOFC was not to be delayed even then.