What is the best train to take to get to Florence from Malpensa airport? Is it possible to not change trains?
Need to define best - fastest, cheapest? Need to go to Milan Central and change train there. Plenty of direct trains between the Milan and Florence. See trenitalia.com or bahn.de for schedules.
A quick look showed 22 trains each day direct between Milan and Florence. 18 are high speed taking about an hour forty and 4 are ICs taking about three and a half hours. Take your pick.
PS - IC are InterCity trains - make more stops.
To begin with, can you confirm whether you're arriving at MXP Terminal 1. That will have a bearing on the transportation options.
If you're arriving at Terminal 1, the quickest way to get from the airport to Milano Centrale is the Malpensa Express. There's a rail station right at the airport and you can buy tickets there for the complete journey to Florence. One point to note is that the Malpensa Express has two routes, one going to Milano Centrale and one going to Milano Cadorna. BE SURE you board the correct train! Also, you MUST validate your ticket prior to boarding the train, or you'll face a hefty fine which will be collected on the spot.
You WILL have to change trains at least once at Milano Centrale (I couldn't find any direct trains on that route). The train from Milan to Florence will be via Freccia high speed train which has compulsory reservations that are specific to train, date and departure time. You MUST only use the train specified on your ticket, or again hefty fines. Total travel time is ~3H:12M. Note that your arrival station in Florence will be Firenze S. Maria Novella (you'll need to know that when you buy your ticket).
As for changing trains: while the Milano Centrale station is huge and on multiple levels, all the tracks are on one level. You simply get off your first train, go the the front, find the big board showing departures, find your next train, and go to that track.
When looking at the board for your train, DON'T look for a train going to "Florence" (or even "Firenze SMN," your station's Italian name). The board shows the final destination, and in smaller letters shows intermediate destinations. Most likely, your train's final destination will be Napoli or Roma. So, find your train by its departure time and train number. As Ken said, you must be on the EXACT train you have a ticket for, or you will get a heavy fine. Don't hesitate to ask for help if you need it. While the station is very busy and bustling, if you show people your ticket, they can point you to the correct track.
If for any reason you will want or need more time to change trains (if you have a mobility impairment or just want to buy some food), make sure when you buy your ticket that you get a later train.
You're set. Buon viaggio!