Quickest train from mxp to Siena?

The website you should be using is Trenitalia (italian Railways): http://www.trenitalia.com/tcom-en
Which are you using?

I can't give a specfic answer because you don't give a time or date. Looking "Malpensa Aeroporto" to "Siena" on Trenitalia for today, I get about 1 train per hour, taking 4h31, with prices in the range €59 to €120, depending on time of day.
There are big discounts for advance purchase, but if you are coming from an intercontinental flight, you cannot predict when the plane will land (+/- 1 hour), or how long it will take to get through immigration.

Keep in mind there would be no direct train. Siena is located off the main lines, so you will have changes, anywhere from 2 to 4 changes, even the train you board at the airport will only take you as far as Milano Centrale, the main train station in Milan.

Overall, the quickest would probably be taking the regional train from the airport to Milano Centrale (about 1 hour), then catch a direct fast train from Milan to Florence (Firenze S. M. Novella, about 1 hour 40 minutes), then to Siena. For the leg to Siena, I would consider the bus, the bus station is adjacent to the main train station in Florence and drops you off in the center of town in Siena, the train is about the same (about an hour and a half) but leaves you at the train station outside of town, so best depends on where you are staying.

Total time will depend on those connections, if you happen to catch a train right away or have to wait a half hour or more for the next one.

Tickets for the fast train from Milan to Florence would be best bought well ahead for price, but if arriving on a trans-Atlantic flight, it would be risky to choose a specific time.

I use a website called loco2.com that was recommended by a user on this forum. You can create a account for free- they list all the trains- Italo and Trenitalia- it is a very user friendly site- and no 3rd party mark ups.

Happy Travels💫✨

If you’re not familiar with this site, it’s helpful to visualize the various options: www.rome2rio.com.

On the left column, there will be links to the various companies, I.e. Trenitalia or bus companies.

Ann,

As mentioned above, the Trenitalia website is your best source of information. Depending on which train solution you choose, that trip will range from about 4H:25M to over 9 hours. The shortest trips use the route Malpensa T1 > Milano Centrale > Firenze SMN > Siena. The advantage of taking the bus from Florence to Siena is that you'll be dropped in town rather than at the bottom of the hill where the rail station is located. You can easily take a Taxi from the station up to the city if you choose to make the entire trip by train.

If you haven't used trains or other public transit in Italy before, note that you must validate tickets for the Malpensa Express and the train from Florence to Siena, as those are Regionale trains. Failure to do so may result in hefty fines which will be collected on the spot. The segment from Milan to Florence will be via Freccia high speed train, and validation is not required for that ticket, as it's specific to only that train, date and departure time.

It's not usually recommended to buy advance tickets to connect with incoming trans-atlantic flights, as if the flight is delayed for any reason and you miss the Freccia train you've bought tickets for, your tickets are worthless and you'll have to buy more at full price.

If you do want to buy advance tickets for other portions of your trip so save money, I'd highly recommend www.trainline.eu as it's very user friendly, and sells tickets at the same price as the rail networks. Note that you'll only save money on tickets for express trains, as Regionale trains are always the same price, regardless of whether they're bought online or at the station.