So my grandfather served during WW2 under Gen. Patton as a locomotive engineer. He died before before I was able to have a relationship with him but my father told me that he relayed this story to him before he passed. Basically the claim was that train operators weren't required to where uniforms and that Patton being Patton got overly aggressive and ordered the line muntitions to advance to the next town, which coincidentally, had yet to be taken by the allies. My grandfather claimed that the German soldiers were rather excited to see that their ammunition was safe and cheered at the site of the safe (American) ammo. Basically is there any plausibility to this story? Or was my grandfather making up a story to quell my father and uncles curiosity? I know the war changed him and that he saw things that he would never talk about.
If the anecdote about an ammunition train actually occurred, it doesn't occur in the U.S. Army Official history (aka green books) that I recall. I double checked the Logistics section addressing railways and it's not mentioned. I am pretty sure that it's folklore, perhaps authentic to the US Military Railways service. So your Grandfather may not have "made up a story" but this might be one he heard.
Furthermore, several exact details are a bit implausible:
He served under Patton. Patton was an army commander (1 out of 6 on the western front) and the US Military Railways service was part of the Communications Zone (COMZ) command which was under SHAEF (essentially Eisenhower) directly. Patton's staff would only be able to request movements of trains, not order them. Patton's staff (among others) frequently conflicted with COMZ in trying to retain loaded rail cars in forward areas so they would be mobile if needed, whereas COMZ wanted them unloaded quickly so they could be kept moving. So there is perhaps a grain of truth in the story, in that Patton might have liked to be able to control train movements in Third Army area, but his staff wouldn't have been able to.
German army units could use American ammunition and would have been happy to possess it. This just isn't true, the shells etc. were not compatible with German weapons.
Not wearing uniforms. In general US military personnel serving overseas wore uniforms. Military Railway Service Troops underwent basic training and had officers and normal military ranks. The military Railway Service also had a military police arm to prevent pilferage (and MPs were notorious for enforcing uniform rules in the European Theater.) Again, there are grains of truth in this story in that the Military Railways service selected it's personnel from civilian railroad workers and had close ties to civilian railroads (each corporation sponsored a unit.) but they were military personnel and subject to military discipline. Also, Military Railroad Service troops worked closely with French civilian railroad workers, often manning trains with joint crews because of the shortage of Railroad military troops. So it's fairly possible that military discipline was relaxed at certain times, but obtaining civilian clothing in the ETO would have been difficult and against regs.
Trains could advance ahead of troops under Patton. The general condition of French and German railways in the western theatre was extremely bad after Normandy. Allied bombing and German demolitions had left the rail network in a very precarious state and the allies were seldom able to make repairs fast enough to keep up with advancing troops. Only when the front lines stabilized for a few months in the fall/winter of 1944 were the rails able to catch up to the front.
In any case, the specific story is probably military folklore - a story that soldiers told each other to break up the boredom common to military service, but it's interesting anyways because of the underlying subtext.