I must say at 32yo I kinda feel the same way as "x-press" in this situation, though I guess this is all just a sign of the times.
But I will say this at least. Over here on "Silver Service" if you ever are (or have been) asked to politely to remove your hat/ballcap when in the dining car, you are most likely in the process of meeting (or already have met) me! OBS...
OBS... I'll be the one wearing the Red Sox ball cap in the diner on the Silver Star the week after labor day.
Actually, I'll probably have the cap in my hand as between my mother and the US armed forces, hat etiquette has been pretty much been hard wired into my brain.
As to clothing customs and etiquette in the pre-Amtrak, pre-airline days, I'd be happy to talk to my grandmother and father about this. My grandmother is well into her 90's. She and my grandfather were the first members of my family tree to make it to the middle class. She rembmbers the great depression and world war 2 like it was yesterday. Always the social butterfly, she would know the fashion rules and customs of train travel back when it was the only way to travel.
My father worked for the Atchison, Topeka & Santa Fe Railroad in Kansas City as a young man in the 1950s. I'm not sure what he did for them or even if he had any passenger contact, but there is no harm in asking. He might also have some insights.
They are both on the west coast where it is a little before 7:00 am. I'll call them later today.
Rick