We travelled from the Algarve to Seville by train and bus. We scheduled a night in Tavira so it would not end up (what appeared to be) a 6 hour trip. It turns out that it's a 1.5 hour train from Estombar-Lago station to Tavira if I recall. It's the train line that runs across the Algarve through Faro, Tavira, etc. Then it's a 8 minute taxi ride to the bus station, then a 2 hr 20 min bus ride - non stop to Seville via Alsa.
What makes it confusing is that there is no train across the border there, so you have to combine bus and train - and apparently the train station and bus station are a few miles apart. and also there is a an hour of time difference which makes it appear that the bus trip is much longer than it actually is.
Tavira is cute and pretty but small and pretty boring. If you just wan a quiet place it's a goos stop. If we do again, we would not stop there at all.
We also made the mistake of paying 7 eu for the little mini-train ride all around Tavira - not only boring, and a waste of time, but I would be careful if you just ate or have. a bad back with so much shaking. The only excitement was when a car was parked illegally and the train was not able to make a turn for 15 minutes, which added a little drama.
The bus is super efficient. Non-stop. It has little airline seat tv's which show movies or updated map locations. (No onboard toilet for the 2+ hour trip so make sure you are ready for that).
Another tip is that the bus stops in the Plaza De Arma bus station in Seville which is by the river. But if you stay on it, it proceeds to the Santa Justa main station another 15 minutes later.
I just thought I would share my experience. It's fairly easy once you have been through it.