The train from termini Station to Castelgandolfo only takes about 30 minutes or so. Here is a some valuable information taken directly from the website santasusanna.org
To reach Castel Gandolfo: Tour Busses organized by Carrani Tours, www.carrani.com ; They leave at 8:30 AM from their office at Via V.E. Orlando, 95 (across from the St. Regis Grand Hotel, and around the corner from Santa Susanna). Carrani charges 45€ per person. You can call them at (+39)06.474.2501. They return to their offices at the end of the audience. By public transportation: (1) by train from Roma Termini station www.trenitalia.com There is a train that leaves Roma Termini at 08:37 and arrives at Castel Gandolfo at 09:17. Then there is a good walk up the hill to reach the Papal Castle. There is a return train at 1:42 PM and 2:27 PM. Cost is 1,90€ for one way;
(2) by Metro and Cotral Bus: Take the Metro (Cost: 1€) to the Anagnina Subway Station, exit on Left, find the Large Booth that sells tickets and buy your ticket to Castel Gandolfo/ Grottaferrata; the cost is of 1,20 €. Then find out which gate the Cotral bus leaves from to go to Castel Gandolfo/Grottaferrata. There are buses that run at 8.30, 9 and 9.40 AM. The trip takes roughly 35-40 minutes. Get off at the Palazzo Papale. The return buses run at 12:50, 1:30 and 2:00 PM. Cost is 1,20€ each way.