http://www.thinkspain.com/news-spain/27109/new-metro-line-9-south-links-barcelona-city-centre-to-airport-terminal-1

which station in Barcelona does this train go to?

It is a Metro train.... So all of them. Just change at a station for a different line/route as you would have on the London or any other Underground.

Hi Pat,
This link may make things a little clearer

.
https://en.wikipedia.org/wiki/Barcelona_Metro_line_9

Thanks for that but studying the map it is hard to see which station to change at for the train down the coast to Tortosa/L'Aldea. In fact I don't think it is possible from line9, so the old route to Sants is still the only answer.

You still catch the mainline surface train to Sants bu then catch the Metro to El Prat instead of that previous horrendous journey with all the hanging around at Sants etc.
As per the article :-

"One airport worker, who was among the first to travel on the 14.55hrs train using her T-Mes card, says being able to get to T1 directly by train was 'a luxury'.
“Beforehand, you had to catch the metro to T2, and then take a transit train which meant another half an hour's travelling,” she revealed.
Until now, travellers from outside Barcelona arriving by inter-provincial train at the main Sants station had to either add an hour to their journey to get to T1, or take a taxi costing upwards of €20.

I dont think the metro L9 will be of much use. Assuming travelling to UK from T2, you will need metro L1 from Sants to Collblanc and then change to L9 at Collblanc for the airport. Gets there after 12 stops.
It probably will be no quicker than the surface train

Also, I Forgot to say I always buy a ticket at L'Aldea to barcelona airport. This is same cost as a ticket to Sants