Avinguda Tibidabo is a metro station. Eurail is not valid on the metro, you'll need other tickets:

https://www.tmb.cat/es/tarifas-metro-bus-barcelona

Montserrat is on the FGC network and passes are not valid there either.

There are combined tickets available for a day return to Montserrat, including the rack railway up to Montserrat (and some other things, see the ticket descriptions):

https://vol.cremallerademontserrat.cat/bitlletes-combinados

Within Barcelona, Eurail is valid on Renfe trains, including the Rodalies.