Randall, your actual topic is "Pompeii Daytrip from Rome". I mention that because you should use the search box at the top center to read many, many past ideas and comments about this frequently asked question. You are going to spend the whole day on this trip, but many people want to do exactly that. It is a major, but entirely practical excursion. It is not a stop along the way.
The bag check at Pompeii is not in the tiny commuter rail station but at the ticket booth complex. I didn't need it, but many readers have reported that it was safe and convenient, over a counter with humans.
Most visitors spend two to four hours at Pompeii, some spend all day. Heat and sun can be a big problem. There is a snack bar inside the excavation, which is important because you can't pass-out and return on the same ticket.
The train line to Pompeii (continuing on to Sorrento) is on the lower level of the same, newish, modern rail station you'll arrive in at Naples. It can be very noisy and crowded. Your luggage won't fit in the tiny racks over the seats that you can't get to because the car is full!