There is no road to Vall de Núria. The only access is the Cremallera de Núria rack railway, and the only way to reach the rack railway is by conventional train — or on foot through the gorge, which is a serious undertaking.
From Barcelona: Take a Rodalies (suburban rail) or regional train from Barcelona Sants or Passeig de Gràcia to Ribes de Freser, changing at Ripoll if necessary. The journey takes around two hours. At Ribes de Freser, change to the Cremallera; the station is adjacent to the mainline platform. The rack railway runs via Queralbs, takes about 45 minutes, and terminates at the sanctuary. Trains run several times a day throughout the year, with additional services in peak season — check the FGC (Ferrocarrils de la Generalitat de Catalunya) website for current timetables.
Combined tickets: FGC sells combined tickets that include the Rodalies train from Barcelona, the rack railway up and down, and optionally activities such as the chairlift, archery, or horse riding. These represent reasonable value for a day trip and can be booked in advance online.
By car to Ribes de Freser: If arriving by car, drive to Ribes de Freser or Queralbs (where there is a larger car park) and board the rack railway from there. There is no parking advantage to stopping at Queralbs beyond saving a few minutes on the rack railway — the scenery in the lower gorge is the best part of the journey.
On foot: A historic path, the Camí Vell de Núria, climbs from Queralbs to the sanctuary in about 2.5 hours through the Gorges del Freser. It is a genuine mountain trail, not a paved track, and requires appropriate footwear and fitness. Many walkers take the train up and hike down, or vice versa.