In the summer, I've found that a T-shirt and basketball shorts works well for me on Amtrak, as seen two years ago: https://www.amtraktrains.com/threads/july-odyssey-cascades-coast-starlight-california-zephyr.83803/ . I did hot-weather pants (Columbia Silver Ridge) on an ICE from Frankfurt to Berlin last August, and they also worked well with the bad A/C on the train. On Brightline in Florida summers, heavier shorts are also fine, but my coastal Florida slacker roots are going to dominate.
My December trip south from DC to Miami on the Silver last December had me mostly in 501s, a T-shirt, and a sport coat when it was a bit cool that first evening.
My December trip south from DC to Miami on the Silver last December had me mostly in 501s, a T-shirt, and a sport coat when it was a bit cool that first evening.