Can you get on a different time train than the ticket you have?

You must date stamp only trenitalia's regional trains tickets not purchased on-line. It takes 2 seconds and you do it just before getting on the train. The machine will print date and time on your ticket and then it will be valid on any regional train on that route for the following 4 hours.

Discounted trenitalia tickets for all other trains (that have only reserved seats) come with restrictions. The really cheap ones are strictly "no changes, no refunds".
Even full price, on-day tickets (called BASE) can be exchanged only up to one hour after the departure of the booked train. If you buy a full price ticket on-line and your flight is delayed for more than 1 hour, you have wasted your money. Hence nobody purchases full price tickets in advance, most get them either at the counter or at ticket machines or via trenitalia's app a few minutes before the train departure. Trains don't usually sell out, it happens only in August and on Easter Monday.

You can get all kind of train tickets also at travel agencies that display trenitalia's logo on the window, while many tobacconists sell regional trains tickets. They can add a small fee, ask before using their services.

I don't know italotreno's rules, it's a private competitor that runs only high speed trains on major routes. I guess their system must be similar: you trade money for flexibility.

On faster trains (Freccia or Intercity), you can buy up to 120 days in advance in any of three fare schedules: super economy (no changes of any kind); economy (some changes with a fee that usually brings the ticket to the full price); and base fare, the full fare price (no advantage in buying early).

So, if you are sure of your itinerary, you can save considerable money by committing to a specific train (date and time), otherwise you may want to keep flexibility and buy tickets the day of (or day before) departure.

It may help to understand why you stamp tickets. The purpose is to prevent you from using the same ticket twice. If your ticket is for a particular train at a partiicur time, then you can't use for anyother time and time stamping the ticket would be redundant. If your ticket is valid for more than one train than stamping is the only way to prove you aren't just using the same ticket over and over. That said, you can see why you wouldn't want to stamp too far in advance as then it would look as if you might have boarded a previous train. The stamping machines are at the entrance to the tracks and very handy. Just stamp as you enter the platform. Simple.

Here is a good primer for all things Italian train travel including where, how, and when to buy tickets. http://www.seat61.com/Italy-trains.htm

Other than the need to stamp the ticket, a lot of the complexities of Italian trains do not (or need not) apply for short trips. Between Florence and Montecatini, for example, only regional service is available. The current fare is 5.50 euros, and that is what you will pay on the day of the trip or ahead of time.

Always look at the trip duration and price before automatically opting for the fastest choice. If you're starting or ending a travel leg in a small city (are you traveling Montecatini to Milan or vice versa?), part of your trip is going to be on a regionale anyway. Be sure you are saving enough time on the Frecce+Regionale combo (compared to all-regionale) to make the extra expense worthwhile.

Edited to add: The wrinkle that we often forget to mention is that if you wait to buy your ticket on the day of travel (which I often do) and don't plan to use a vending machine, you must allow time for there to be a line at the ticket window. You cannot count on the line to move rapidly. I missed a train while a lengthy personal conversation took place at the ticket window. That could have happened anywhere--all it takes is two discourteous people, but the ticket lines in Italy just seem a bit less efficient than what you'll see in a lot of other countries. Don't cut it too close.

Regionalle tickets: If you buy these at the station they do not have a time in them. As said above, you then need to timestamp ("validate") them before you get on the train and then they are only valid for a short period. You can buy them in advance on the internet, in which case the come with a date and time already on them. But, there is little point in doing so as there is no discount for advance purchases.

Long distance and high speed trains there is a big discount for advance purchase. But, these tickets all have a date, time and train number on them, and are only valid on that train. If you are not sure when you want to travel, best to just buy at the station on the day and pay the full fare ("Base" fare).

The above is for Italy. Slightly different rules for each country, but in general if there is a date and time (and usually train number) on your ticket, hard luck. Same as plane tickets, only valid on that train.

Just in case you've been digging around in old info, there was a fairly recent change to regionale ticketing. You used to be able to buy them at the station up to two months in advance for a specific journey and use them any day you wanted to. Now, you have to specify the DAY you intend to use them if not the time. This should be helpful:

https://community.ricksteves.com/travel-forum/italy/regionali-train-tickets-important-changes-effective-aug-1-2016

I generally fly to Milan, take the bus to Centrale, and go by train to my final destination. My strategy is to buy a super economy ticket (roughly $10) in advance for a train 4 hours after flight arrival Worst case scenario, which hasn't happened yet, is that I am out $10 if the flight is delayed and I have to buy a new ticket. More likely outcome is considerable savings and time for a nice lunch in the Asian restaurant at Milano Centrale.

If you do buy one of the nonrefundable fares on the fast trains with reservations included - by all means do NOT jump on a different train and attempt to use your ticket as a group of senior travelers did to us a couple of years back. They had tickets/seat reservations for a different train, but decided they'd rather be on our train. They took our seats and caused considerable stress to us and everyone around us who had to endure our standing in the aisles with luggage - all because they didn't know (and refused to believe) that their tickets were NO GOOD on our train. We finally pried them from our seats after much stress. The train was so packed because of the fiasco the ticket checker couldn't even get down the aisle.

What a nightmare.

To add to Tim's story. About a year ago, on a TGV waiting to leave a Paris station. On the opposite side of the aisle to me were two people speaking American English. A man and a woman, but from how they were talking, obviously a business trip. Three Koreans tourists (I guess) got on before the train started, and politely said that one of the three seats listed on their ticket was where the man was sitting. The man insisted said he had a ticket for that seat. One Korean had to sit at the other end of the coach.
Some time later, after the train was underway, the conductor comes round to check tickets, and informs the business man that yes, his tickets were for this train, coach and seat, but on a different date. He then charged the painfully high fare for not having a ticket, and sent the two passengers to other seats, so the Korean could re-join his fellow travellers.
The surcharged traveller could later be heard on his phone complaining to a colleague that this would never happen in the US, as they check all tickets before you get on the train.

MORAL: Always double check your ticket is for the train you think it is.

I always purchase our train tickets months before the trip to obtain the cheaper tickets. Those are for trains that we will be taking between cities where we will be staying, and for us, we like to leave a town around 10:00am to arrive around noon at the next location. I like the convenience of having our tickets ahead of time. We don't need to arrive early at train stations when we're departing and less likely to have any potential pick-pocket train station issues because we're not using money or credit cards in the train station.

For train tickets that are Day Trips, i.e. not sure of which day we might want to go during a 4-day stop, we buy those at the train station. They're usually a 30 min. or 1 hour train at the longest, so the price isn't a big deal to have the flexibility.

liz,

I haven't read all the replies, but a quick summary.....

• If you buy tickets for Regionale trains at the stations, either at a Kiosk or staffed ticket office, you must validate them prior to boarding the train on the day of travel. You can buy tickets before the day of travel, but just don't forget to validate them on the day of travel. Failure to do so may result in hefty fines, which will be collected on the spot. If you buy Regionale tickets online, these come pre-validated but must be used within four hours of the stated departure time.
• if you buy tickets for the "fast" trains such as the Freccia or InterCity, these will include the compulsory seat reservations. The tickets (and reservations) are specific to train, date and departure time so can only be used on the ONE train listed on the ticket (ie: ES-9412). If you board any other train, even by mistake, you'll again be subject to hefty fines which will be collected on the spot.
• With tickets for Regionale trains, there are no discounts for these so tickets can be bought any time and the price will generally always be the same.
• With tickets for the fast trains, you can save considerably with advance tickets if you can get the Super Economy tickets, however you must be willing to commit to a specific departure. Those advance tickets are non-refundable and non-changeable once purchased so if you decide to change your travel time, you're out of luck. You'll have to buy new tickets, probably at Base Fare (the highest price tier).
• Regarding validating tickets. You should only validate just before boarding the train on the day of travel as the tickets have a "shelf life" once validated (about 4-6 hours, depending on route). If you were to get off a Regionale train to have a look at a particular town and then re-board the next train more than four hours since the ticket was validated, your ticket may have expired and you guessed it, hefty fines on the spot! Tickets are relatively cheap for the Regionale trains, so it's often easier just to buy a new ticket. You may find this website helpful - http://www.seat61.com/Italy-trains.htm#Travel%20tips
• If you buy tickets for the competing Italo Treno high speed trains, you must also have compulsory reservations for these, so consider the rules the same as for Trenitalia.
• Note that many cities have more than one rail station so you must know exactly which stations you'll be using when you buy tickets.

Chris F: People who just won't listen to advice or common sense deserve the highest fine possible (IMHO)!

And if the tickets HAD been checked before he got on the train, that passenger would probably have been on his phone complaining about the inefficient railroad ticket agent who sold him a ticket for the wrong date. Because it couldn't be HIS fault.

Could be that the passenger travelling on the wrong day mixed up his dates.
In most of the world, 9/11 is the ninth of November.

Well, I still remember a group of obnoxious travelers (they were sport fishers back from a competition) that had wrong tickets and got so offensive with the conductor that they found police waiting for them at the next stop. While this is a very unfrequent option, it is still an option.