I'm planning a trip to Rome in mid May. I would like to take a day trip to Positano even though some people don't recommend this. I realize my time will be limited. It will be me, my wife and two girls. I think my best option might be train to Naples then car service to Positano. The question I have is, I see the rates around 110 euro. Is this one way or round trip? Also what time is the latest they will return to Naples? Another consideration would be an overnight stay in/around Positano.
We paid that amount one way from Sorrento to Naples airport so assume it’s a one way price.
Where are you seeing rates of 110€? The trip is certainly possible, but it will be a long day. Try to leave Rome as early as possible and return in the evening, so that you have as much time as possible in Positano.
You might try contacting THIS firm and asking for a quote.
You could also travel to Naples by train, transfer to the Campania Express to Sorrento and then have the hire car waiting for you there. That will make the distance to Positano a bit shorter (and cheaper). I haven't checked the schedule for the Campania Express, but it should be operating in mid-May.
You could train Rome-Salerno then ferry to Positano. Not quite as expensive as a car service. But... whichever way way you decide, keep in mind you'll spend about 5 hours R/T in transportation for 4-5 hours in Positano. Look carefully at train times and connecting ferry times.
https://www.trenitalia.com/tcom-en
https://www.positano.com/en/ferry-schedule?from=salerno&to=positano#