Yes, taxi is the most expensive option for sure, although prize is according to distance from El Prat- Airport to destination and the quantity of baggage. Aerobus is not a bad option, because is frequent and a express bus with few intermediate stops, don´t cross El Prat city, and run from all two terminals to Plaça Catalunya square. But is a private service, and subsequently with their own expensive prizes, and not an integrated transport. The most cheaper is L-46 normal bus by Barcelonian TMB, because is integrated. Only run from all two terminals to Plaça Espanya square. But because is integrated is possible to take TMB and FGC Metro or other TMB urban bus lines with the same ticket. It´s less frequent that Aerobus, and slower because cross El Prat city and it makes more stops along Gran Via avenue in L´Hospitalet and Barcelona cities. TMB Metro L9 is perfect in the case that your destination in the city is near one of the stations. But in case of interchange with other lines is less competitive. Prize is double in T-2 and T-1 stations, but tickets are integrated the same that TMB bus. And finally, a new subway commuter line, or Rodalies in Catalan, is just finishing construction, with stations under all two terminals. In the near future will be exploited by Catalonian FGC with very frequent service and, subsequently, this actual Rodalies branch that only serves T-2 in surface by Spain´s RENFE simply will be closed.

I think you can. www.barcelona-tourist-guide.com/en/transport/barcelona-metro.html#metroticket

Now is the T-Casual

Yeah, the railway station is at the Terminal 2.

How long is the walk from terminal to train?

looks like terminal 2 ngl