Train between Seville and Ayamonte?

I don't think so. Assume you check the renfe.com site? The rail system in Spain is not as extensive as the rest of western Europe. But they have an excellent bus system that is fast and cheap. There will be a bus.

There's not even train tracks west of Huelva down that way.

Thanks. A bus it is.

The bus system is great and should not be overlooked in Spain. We ride the buses about half time when we are in Spain.

No trains to Ayamonte. Good bus connections from elsewhere in Spain. Are you making a run for the Portuguese border? We did that (coming the other way, from Vila Real de Santo Antonio, Portugal). There's a little foot-ferry across the river, connecting the two towns. On the Portuguese side, there's no passenger trains to Vila Real de Santo Antonio anymore - there are tracks and (maybe) freight trains, but when I was there a few years ago there was no longer passenger rail service (there once was). Good bus connections to the west, though.