Well if you look a lot of those threads they might start off and end different but the original question was answered but then the thread took a side track. Some do go astray like you said, and I probably do it myself at times.

But eventually I will get back on the main line again.
Others do, like you said.
That said I think your engine is number 391. 1938-1942
It was the same body style used for 396 and 397
A picture,
I found that here,
http://www.ebay.com/gds/MARX-TIN-TRAIN-LOCOMOTIVE-IDENTIFICATION-GUIDE-/10000000009480012/g.html
check it out there are similar locomotives but with letters or other stuff different.
There is not much on sets there. But info on other Marx trains.
Now I think yours might have come with a 951a tender shown below.
The only reason I think that is because I found this for sale. It is your locomotive above right? It is shown with this tender. Now the question is did this tender come with this locomotive?
Take a look at the listing there are more pictures there that you can enlarge.
http://www.ebay.com/itm/ws/eBayISAP...3080&item=221350133080&lgeo=1&vectorid=229466
Now this listing came from here,
http://www.vintagetoysauctions.com/MarxTrain.html
They have a bunch of Marx trains there I did not look through the whole site maybe there is a tender listed there. I have limited time to be on the site right now or else I would have looked.
I can't seem to find a site for info on sets alone.
Maybe your set originally came with a tanker instead of the extra gondola car?
Maybe someone switched it, it seems odd to me that a set would come with 2 of the same car instead 1 of different cars.
They do sell the Marx ID book for trains you know?
Maybe they have info on the sets in there?
Maybe someone here has one and can look?
I will keep my eyes opened for Marx sets.
Have you found any sites listing sets so you can definitely see what came with your set?