Florence to Lucerne by Train

Should be able to do one ticket. Basic question. Which web site are you using to book the ticket? Then we can tell you how to do it.

Well, I'm in the midst of an internet outage, so I'm not entirely sure (I'm doing this from my phone). I think I was on trenitalia (?). Or maybe it was some European rail site.

It is hard to get Trenitalia to recognize Luzern as a station, as most journeys involve a transfer from the main Milan to Zurich route at Arth-Goldau. The short leg from A-G to Luzern is within Switzerland and cannot be purchased on Trenitalia.

Try Firenze to Arth-Goldau instead. You should see a number of choices, with a single change at Milano Centrale. You can buy the short hop from A-G to Luzern at the station when you get there.

It appears Trenitalia can't cope with changing train outside Italy. It only shows trains which are direct from Milan to Luzern.
Try using www.sbb.ch (Swiss railways) to find out what trains exist, that site will show all trains including cases where you need to change in Switzerland (which is the majority of trains).

Make sure you spell placenames to local way, Firenze and Luzern.

P.S. "I think I was on trenitalia (?). Or maybe it was some European rail site.". Personally, I do not part with money to a website if I don't know who they are. In the case of train tickets, they are usually cheapest when purchased directly from the railway companies (in this case Trenitalia or SBB). Third party sites have price markups and do not offer all options. And Trenitalia is a European website.

Got it! Thank you both so much.
Is raileurope.com the way to go, or is that one of the middlemen I should avoid?

Book directly with the rail sites. RailEurope is a travel agency that re-sells rail tickets. They do not offer tickets on all the trains and they do not discount tickets. Some like RE for the high service level but your pay for it.

I just booked the opposite direction--Lucerne to Florence--yesterday for an early March trip. I was advised to split it up into separate tickets (Lucerne/Milano Centrale, then Milano Centrale/Florence) and it worked fine. Used the SBB (Swiss) website for the Lucerne/Milan leg and it went flawlessly, got deep discounts and printed out E-tickets--2 people for $41/pp, with one change of train at Arth-Goldau. However, when I tried SBB for Milan/Florence, it was $85/pp, which seemed high for advance tickets. So I went to Trenitalia, where I got them for $23/pp! I printed everything out & I think I am good to go, although the SBB website is so much cleaner & efficient online than navigating through the somewhat-vague Trenitalia site. Swiss vs. Italians I guess!

Of course, the only problem with these 'Super Economy' tickets, SBB or Trenitalia, is that you must take THAT train on THAT day, no deviations. But I figured that they were so cheap that it's worth the risk, and we kind of know where we're going on each day of our trip.

Good luck!

European web sites don't sell tickets for connections outside their respective country, so the solution for multi-national connections always is either to

a) purchase legs through separate web sites

b) purchase at a train station in Europe (which can handle more international options)

c) book through an US agent like Rail Europe. Rail Europe does sell discounted Italian train tickets, as well as discounts for several other countries.

Regardless of source, this route will usually be booked as two separate tickets and reservation and refund rules on each leg will vary, depending both on the type of train and the type of fare paid.