trains for Rome to Salerno to Cavriglia to Rome

Why would you need a pass saver? I am not following the question. I don't believe Cavriglia has train service.

It is always more sensible to buy train and bus(which you will need to get to Cavriglia) tickets, rather than an over-priced pass in Italy. You just have 3 easy train journeys and a local bus to take.
Get the bus to Cavriglia from Montevarchi train station.
Train schedules and prices at
https://www.trenitalia.com/tcom-en

For some reason, the marketing of "rail passes" has been so pervasive, not only do new travelers to Europe think they are still a good deal (most of the time these days, they aren't), but that they need a "rail pass" to ride trains at all.

Pat, for any train, you can always just buy an individual ticket (sometimes called a "point to point" ticket, to distinguish it from a pass). You don't need a rail pass to take a train, any more than you need an "air pass" to take an airplane.

The particulars of buying and using train tickets are different for each country.

For Italy, you can buy high speed trains in advance online for a discount, but these tickets will be non-refundable and non-exchangeable (or have significant restrictions on these actions). Or, you can buy tickets just before travel, at full (Base) price. In Italy, the Base price isn't too high (unlike some countries, where it be very high). The high speed trains are the ones called InterCity or Freccia ("arrow" - FrecciaRosa, FrecciaArgento, etc). A seat reservation is included with your ticket. For these trains, you must take the EXACT train on your ticket (not the one an hour before or after), so only buy tickets once you know your plans.

For Regional and Regional Veloce trains, there is no discount for advance purchase, and tickets can be bought online, but these have more restrictions than Regional train tickets bought at the station. These tickets are good for the day you specify, on any train that day. However, they MUST be validated before travel in the machines in the stations (this prevents re-use).

More about Italian trains here, from the rail guru The Man In Seat 61: https://www.seat61.com/Italy-trains.htm

Thanks for all the informative replies!