voltarre to riomaggiore

Probably both. First the SITA bus from Volterra to Florence with a bus change at Colle val d'Elsa or Poggibonsi. Second, the SITA bus terminal is about 800' from the Florence SMN train station. You will need to walk over to the train station and take the train to Riomaggiore. About 4.5hrs.

There's also a more direct bus-train route via Pontedera. Bus #500 goes from Volterra to Pontedera with no change required (same bus line – CPT). The train station in Pontedera is only a block from the bus station and an easy, flat walk. If you take the 10:05 bus from Volterra and connect to the 11:49 train (24 minutes between arrival and departure), you can get to Riomaggiore in about 3.5 hours. The earlier and later combinations take about 4.5 hours, but I found it faster and easier than via Florence. My actual trip was from Cinque Terre to Volterra, but going via Pontedera saved me at least an hour of travel.