Trainguy Key...I looked in my Marx reference material and in short found no perfect match to your set. Here however is what I did find.
The engine is a 591 windup and it is accompanied by a 551 wagon top, NYC tender. This combination was made between 1950-1958. Note that the engine also came with an electric motor and even in wind up had many variations (9) but yours with both side rods and hand rails is one of them. Further dating might be possible if you would state whether or not your locomotive and a headlight and / or a sparkler.
In terms of the set, the closest I came was set # 9507 (B) which appeared in the 1955-56 Sears catalog. This set came with 0-27 2-rail track to make a figure 8 (thus it would have had the cross over), instruction sheets 212 and 71 and a key. It sold in the Sears catalog those years for $3.97. This set had all your cars with one exception. It had the high sided, 28500 LV green gondola as well as the 554 red one that your set has. However, this set did NOT have the 738701 PRR hopper car. However the variation of this car that you mentioned, with tab /slot couplers and plastic wheels is listed and this car was made from 1940 onward so it is certainly appropriate.
My guess is that sometime in its 70+ years of existence, someone either lost or broke the LV high sided gondola and replaced it with the PRR hopper. After all, these cars were sold in the 1950's via 5 and dime stores for about 25 cents apiece.
Its a GREAT SET, congratulations. Finding anything this old and still more or less complete is a wonder for sure. That happens to be especially true of Marx because as the "low end" leader, parents etc were more prone to throw them away after a few years around the Christmas tree vice expensive Lionel's that were carefully protected between Christmas seasons.
Before I forget, Daniel that is a beautiful LR set that you posted. To me, one of the interesting things was that I observed that the picture on the LR box was what was actually in the box!! We all know that toy train manufacturers (Hornby was one of the worst) greatly exaggerated what was in the box through the picture on the cover!!
Best Wishes Don
