I'm just wondering though, even if the AI won't trade their only resource, should these single resources still be showing up on the trade screen (but reddened out)? Because for some reason, it's not showing up on my trade screen.
if the ai has 1 source of iron, it won't show up in the trade screen. if they have 2, it will show, but it might be redded out for various reasons. but if they then trade that second source away to another ai, i
believe that it disappears from the trade screen until that deal ends or they find more resources.
wrt op, you need a direct route to their capital. so rivers and coasts (w/ sailing), or ocean (astro), or a direct road (wheel) will all work, as will combinations of all three. the ai will eventually connect all of its cities via road networks, so just connecting your capital to one of their boarder cities will in turn create a path to their capital, eventually. if you have open borders, you can even send in your own workers to build roads, but i think this would be a waste of your time in 99% of cases.
there is one weird exception to ocean trade routes pre-astronomy. if your culture goes out into the ocean, but borders the ai coastal tile (still water) you can trade resources and open trade routes even though technically you are crossing over a string of ocean tiles. it looks sort of like this:
LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL
ccccccccccccccLcccccccccccccc
ooooooooooooocCcooooooooooooo
ooooooooooooocccooooooooooooo
ooooooooooooooooooooooooooooo <= not entirely impassable ocean tiles
ooooooooooooocccooooooooooooo (pre-astro)
ooooooooooooocCcooooooooooooo
ccccccccccccccLcccccccccccccc
LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL
crude, yes, but i'm at work. anyways, the big C cities are located on the tips of two tile peninsulas. if your culture goes out over the ocean, you can benefit from foreign trade routes (w/ OB) but the ai does not. you can also send workboats to scout their territory early.