There is indeed a good railway between Guarda and Salamanca... but there are no passenger trains on it. :(

Only 2 daily passenger trains until the border but then you're stuck in Vilar Formoso.

Blame the Spanish government/RENFE/ADIF for that. Maybe service will be reinstated in the future but I haven't heard of plans.

For now there are 2 ways to leave the country : Porto - Vigo and Entroncamento - Badajoz. It would be quite the detour but it's surely possible by train. Let me work out a timetable if you're curious.

Via Porto - Vigo it would take 13 hours in total with a 5h30 wait in Vigo... far from ideal. Not better in the other direction with a 7h wait

Thanks much; strange that on all the rail booking sites they couldnt just throw in a blurb about that line being inactive.  Instead they say “can’t find any trains” which seems to imply if you change your travel dates you may connect to one.  Thanks again