There are a couple of different options for travel between Pisa and Genoa; you can either travel via the Cinque Terre on a Regionale train, or take a faster InterCity route which travels along the same route but stops less frequently. The Regionale train described here runs less frequently direct to Pisa, although there are plenty of services travelling to La Spezia (around 8mins from Riomaggiore) which has plenty of onward services to Pisa. In fact, there is only one direct service between Genoa and Pisa each day, and only travelling to Pisa. If you want to travel at any other time, or in the opposite direction, you will need to catch an InterCity service, or travel indirectly, via La Spezia.
This route is operated by single decker trains (as opposed to the double decker services found on other Regionale routes). Between La Spezia and Genoa, the journey passes along the coast, stopping at the Cinque Terre (Riomaggiore).