Train from Pisa to Milan

A search on Trenitalia.com shows several trains per hour along that route, so you’d be fine just showing up at the station.

But if you want a direct train, there 6,
8:57 am IC
11:42 am IC
1:42 pm IC
5:04 pm Freccia
5:42 pm IC
7:07 pm Regionale

Getting tickets on the day of travel, with Trenitalia you will pay the top, walk-up "BASE" fare between Florence and Milan. That's 56 € per person in Standard class, € 67 in Premium Class and € 77 in Business.

You should Check also the on-day prices of Italotreno. They are often cheaper than Trenitalia's.

You can't get on Intercity and high speed trains without a seat reservation, it's always included in the ticket and it can be purchased up to 15 minutes before the scheduled departure time.

There are a lot of trains that day, but most aren't direct trains. Most people hate to change trains, and it's not fun with a lot of luggage, but I often don't mind the changes if I have to. If I were going tomorrow and had nothing booked, I see I could go as cheap as 23.75 euros regional trains (with one change) at the crack of dawn or late in the day. These take just over four hours, which is about the same as a direct IC train. A few direct IC trains for tomorrow are still not that expensive if I were buying tickets now whereas some are already over 100 euros per ticket. We have no idea how things will look prior to October 4 at the last minute.

If you don't mind a change of trains, the fastest way to get there is connecting to a Eurostar train in Florence - leave at 8:32 or 10:32, change in Florence (22 minutes should be plenty), get to Milan at 1:50pm. October 4 tickets are 26.2 Euros bought today, who knows what it would cost if wait. The same train for tomorrow costs 70.30 Euros. If you don't want to change trains, there's a 6:42am and an 11:42a train that takes an hour longer than the Eurostar with connection - only 17.90 Euros for Oct 4 if you buy today.

If you are traveling with luggage and you are not pressed for time I would take an Intercity with no changes required. It will take you maybe 4.50 hours instead of 3.5 hours with a Frecciarossa via Florence, but you will save money and you don’t have to deal with lugging suitcases from one train to the next in Florence. The chances of an IC selling out all seats on a weekday in October in the middle of the day are practically zero. October 4th is St. Francis of Assisi’s day on the Catholic Calendar and St. Francis is Italy’s patron saint but that holiday is not observed much.

That 5:04 Frecciabianca is direct and a little faster than the IC trains at under 4 hours.

Thanks to all... love this resource!
-Alison