Arc de Triomf and Catalunya Rail stations are both closed (I think it's until the 23rd August and there are more stations closed beyond Catalunya) this does not affect the Metro.
I boarded a train at Blanes to travel to Catalunya, on arrival at El Clot several security guards boarded the packed train and the majority of passengers hurriedley disembarked so it seemed like a good idea to join them and not hang around trying to ask questions, though once on the platform I caught sight of a notice board displaying a map which showed a diversion route around several Barcelona stations so this calmed my fears that we were caught up in a security alert. Several hundred of us were herded through the station to the adjoining Metro line where we awaited a train to complete our journey, packed Metro train arrives and several hundred of us try to squeeze on, it was not much fun but at least if you know what is happening it may not be so unnerving.
For the return journey simply go to the ticket barrier at Catalunya station where there were several staff to give advice.

andy