There's a little gap at the bottom left and if I tug on the trim, it's loose and hollow. I can hear road noise coming in from behind it.
I just wanted to check if it's safe to caulk here before I fill that gap with a strip of foam, then caulk it off. Ie if there's any purpose to the trim being loose that I'm unaware of.
Edit: this is the interior.
This is the interior of a fixed window. Below it is a sliding screen window, then a sliding single pane window, and finally another sliding single pane window.
Unfortunately since these are single pane windows most of the thermal performance issues will be due to the glass. Sealing this gap will help some, but it will be just a drop in the bucket.
Replacing the windows with insulated glass (two pane) windows is ideal. However that will be expensive.
The easiest/cheapest option will be the temporary cling-film type kits you can buy--you won't be able to operate the windows, but that probably won't be an issue in the winter. Properly sized/installed storm windows will help some too, and be significantly cheaper than new windows.