Going by train from Rome to Castel Gandolfo
I have written a Travelogue that describe in words and photos how to get from Rome to Castel Gandolfo.
Castel Gandolfo is 15 miles (24 kilometers) southeast of Rome and the best way, if you have the option, is by car - then you can drive comfortably around the lake and villages. We returned from CG to Rome on a Saturday evening with no traffic in about 25 minutes.
However, it can be reached by bus from Anagnina Metro station via COTRAL, which would be the next best option, as it does leave you close to the Castel Gandolfo upper city center and the Papal Palace.
Trains are infrequent from Termini station, but the journey is very scenic. The station is halfway up the slope above Lake Albano (be alert as the station is not clearly announced or signed). A steep path zigzags uphill to the town, emerging just below the main square. It's quite a climb.
The train and buses connect the town with the rest of the Castelli Romani.
The enterprising could try walking to nearby Albano (along the wooded main road above the lake; 25 minutes then a steep drop into Albano) or down to the lakeshore and Marino. However, the roads are designed for cars and not pedestrians, so it's not a particularly enjoyable or safe walk.
take METROPOLITANA A to ANAGNINA.
then take COTRAL BLUE BUS to CastelGandolfo.
About 30-40 minutes ride.
Buy the Cotral ticket at newspaper stall in the metro station,
from the exit gate turns right and on the left side, you'll see the stall.
1 EURO for one way ride!
Many years ago, it was possible to take a chair lift from Castel Gandolfo down to the level of the lake, where there was a large swimming area. I don't know if it's still there, but it was a fun ride!