The Jul 1938 issue of Popular Science has some simple plans for a loading platform. While the materials have changed in the last 72 years … the dimensions shown for the platform haven’t. 
I completed the Loading Platform with both ramps (I’m dropping one ramp for my layout to fit in a planned space). In my version the ramps are slightly longer (about three feet) and the platform is 8" wider. That was an accident but since the original was 12’ wide … and mine is then 12’8" wide … and the US Army Corps of Engineers suggest that for a side-loading platform the width should be 20’ … I think that will work fine. 
I re-created the plan in PDF format. You can download from my website (the are scale plans … you could build directly on top of them). I made them for N, HO and O scale.
Thanks for the post. Friend of mine gave me some old MRs, but not that far back. Have been planning an unloading platform for a team track. It will give me some dementions to “get it close,” especially the supporting bracing. Won’t be that long, but one car wouldn’t need one so long.
Thanks again,
Here’s another useful source. This is an Army manual - EM 1110-3-150 titled “Engineering and Design - Storage Depots - Mobilization Construction” (proponent is US Army Corps of Engineers). Chaper 3 is titled “Open Storage” and has a good section on loading platforms complete with diagrams and numbers. The say the length of a Side Loading Platform should be at minimum, one car length and preferably two. They also say the width should be at minim 20 feet wide. Of course, when modeling we often have to ‘fudge’ those proto dimensions. There is also diagram of a Combination End and Side Loading Platform. http://140.194.76.129/publications/eng-manuals/em1110-3-150/c-3.pdf
I likely will not have a use for this but thanks for the post (and work).