As it turns out, this photo is in fact reproduced in the book The Rape of Europa as a photograph of the Polish art historian Karol Estreicher welcoming the Lady with An Ermine back to Cracow. The train would have gone from Nurembreg to Cracow between April 28th and May 1st, 1946 to take the painting back from the collecting point where art looted from Cracow and taken to Germany was stored after its discovery. If you contact the military with those dates you can likely find out who else besides you grandfather was present when the car was loaded or unloaded and perhaps even find anyone who knew you grandfather in the military that way.

Update: My mom figured out that the photo was used as the cover for "Rescuing DaVinci" By Robert Edsel. From there I found a c-span book talk video and in it he talks about the photo and states that the two men in the middle are known monuments men, but the other two soldiers haven't been identified. He even goes on about how someone will come up to him one day to say "hey that's uncle bob". So I'm going to do just that tomorrow - well, attempt to anyway. With help from the monuments men foundation and whatever we can find out from the army, we should be able to gather info to share with the rest of our family. My mom is literally freaking out and is planning on reaching out to local news media about this.

I can't really tell you much about the Monuments Men aspect of the picture, but I do recognize the patch on his shoulder. It's the insignia for the U.S. Seventh Army, nicknamed "The Seven Steps to Hell" because of the shape on the patch. This picture was assuredly taken post war, as /u/farquier indicates, because those are not combat uniforms. There is too much decoration. The two little stripes visible at the bottom of your grandfather's sleeve are overseas stripes, indicating six months of service overseas each, for a total of at least a year spent overseas. Finally, I am 95 percent certain your grandfather was an enlisted man, not an officer, which I can tell from the placement and shape of his collar discs. Note that they are round; the man on the far left is an officer, you can see he has four different pins on his collar, two U.S. pins and two denoting his occupation (which I think is as an MP, it's not clear, but I think those are crossed pistols).

While it may not be as quick as you'd like, I'd recommend requesting his official service record. I got my grandfather's Navy records about two years ago and was blown away by how much paperwork they were able to keep together. They did a surprisingly good job of documenting movements and temporary transfers to other units. Farquier pointed out the dates that train was traveling, there is a chance that the service record lists more info about the train, other people he was with, other units, and so on.

Once you get the service record, pay attention to names and units. Many papers will be signed by either a commanding or junior officer, go through each page and jot down that kind of info. Once you have some units, locations, dates, and names, start googling like a mad-man. I discovered that my grandfather's commanding officer donated a bunch of materials to East Carolina University. Got in touch with their library, they're going to be digitizing the collection (pictures, a diary, unit roster, etc) and emailing it to me. Being involved in the return of stolen artifacts is pretty spectacular, I'd like to think that there is a good chance someone he knew donated materials to a museum or university as well.

Just here to post an update - I'm not sure how quickly things are going on the monuments men researchers end, but my grandmother has shared pictures our family didn't know about.The consensus seems to be that he was involved with many missions for the monuments men and my grandmother has pictures and records to show that. The researcher did state to my mother that he would be added to the list of soldiers that supported the monuments men. He was involved in the Salzburg mine investigation and possibly other missions that were portrayed in the film. This has been a really cool experience, thank you reddit historians for all of the information. Hopefully the documents and pictures we do have will provide historians with even more detail on what it took to save the worlds most precious art.