Absolute perfection again! The Renfe R2N (light green) and R2 (dark green) trains run directly from the centre of Barcelona to the Circuit de Catalunya. They depart from El Prat airport, so even if you've just arrived by plane, you can go straight to the circuit at Montmeló. Just follow the signs. Other R2 lines end before Montmeló. But it's no problem to change to the right line at any time. It takes about 25 minutes from the centre of Barcelona and 50 minutes from the airport. The prices are basically ridiculous - a ticket costs about a unit of euros.
The only downside is that the train has a station in Montmeló, and from there it's a 35-50 minute walk, depending on which station you're going to. However, you can use the shuttle buses.
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