Funicular is a little train. It goes from Paral.lel, next to the underground stop, to Montjuic. Once at the top there if you want you can catch Teleféric and go to the top pf Montjuic, where there is a Castle.
It’s a slowly train and one part of the way is underground. You cannot see beautiful views but is the most ecological and cheapest transport to go to the top of mountain Montjuic, better than with you car.