Point to Point Train Guidance

Rail Europe is a reseller and will cost you more than booking directly with the train company, https://www.trenitalia.com/en.html. Venice to Florence is a high-speed freccia train and if you buy in advance, non-refundable tickets (just be certain you can commit to day and time like you would for a flight) and you will get a significant discount versus buying the day of travel. You can usually buy those tickets up to 120 days in advance. This ticket will have seat assignments and you do not need to validate it. Just show it to the conductor. Note on the Trenitalia website you will need to enter the city names in Italian, Venezia and Firenze.

Siena to Roma you will change in Firenze. Book it as one ticket again on the Trenitalia site. As you have a combination of regionale train to Firenze and freccia to Roma, you can book it as above, in advance, and again, no need to validate as the ticket is issued for a specific day and time. The regionale train will not have reserved seats but the freccia will.

If the Trenitalia site is intimidating or have credit card trouble, you might try booking at https://www.thetrainline.com/.

You do not need long to change trains, but in Firenze I would allow at least 20 minutes as you are (as of now) unfamiliar with the station and having a little more time may make you more comfortable. You can always have a coffee while waiting. ☕️

Your first and biggest mistake is RailEurope. Use trenitalia.com or italotreno.it to book your Italian train tix. No need to deal with RE being in the middle of the things.

Purchasing those tix ahead of time will save you lots of money.

All you need to know here:>https://www.seat61.com/Italy-trains.htm

However not all possible train possibilities for September (9 months away) may be loaded into the trenitalia.com or italotreno systems (thus not into ANY system), yet. You will do fine - and have access to the cheapest seats available - if you book 4 months, or even 3 months before your trip. Example: A good friend booked his Rome to Assisi ticket 6 months ahead, last year, and only saw an 8-hour trip with a connection through Bologna, for 50 euros. He booked it. Two months before the trip, I noted and passed on to him a less-than-three hour trip with one short local connection for under 20 euros. Moral of the story: don't book too soon!

Thank you for the advice!!

Transfers: get through tickets on the official sites and your connections will Always be protected. If you miss a train with reserved seats you must get a new ticket for free at the counter. In other words: let trenitalia.com do its job picking the official minimum transfer time for each station.

The above advice by jmauldinuu may be a little misleading. Since you are travelling in september, We know when soon may be "too soon": before June 15. The Tickets of all trains running between June 15 and December 15 will be available on trenitalia.com starting from June 14, the day of the summer timetable change. On June 15 you can rest assured you are looking at the complete schedules valid up to December 15.

No ticket purchased online on the official trenitalia.com site needs to be time-stamped before getting on. When you buy online a Regionale ticket, the four hours time-window starts with the departure of the chosen train. Stamping date and time on a pdf printed at home would be pointless.

ps,

As the others have mentioned, you may pay more by booking through Rail Europe than by booking directly with the rail networks. Having said that, I often use a reseller for tickets in Europe - www.trainline.com as they sell tickets for several networks in Italy as well as other countries in Europe.... one stop shopping.

The two trips you'll be taking are very easy....

• Venice to Florence - you'll be travelling by high speed Trenitalia (Frecce) or Italo trains (take your pick). The tickets will be specific to train, date and departure time and can only be used on the one train listed on the ticket. If you board another train by mistake, you'll be subject to hefty fines which will be collected on the spot. The ticket will include your seat assignments and does not have to be validated (punched).
• How are you getting from Florence to Siena?
• The specifics of your Siena to Rome trip will depend on which class of trains you'll be using. What time of day will you be making that trip? If you'll be using Regionale trains and buying advance tickets, you don't need to validate them. If you use locally purchased tickets for Regionale trains, these DO have to be validated prior to boarding the train on the day of travel. Failure to validate may again result in hefty fines. There are no seat assignments on Regionale trains, so just sit wherever there's an empty seat.
• Your trip from Siena to Rome may involve a Regionale to Chiusi-Chianciano Terme and then an Intercity from there to Roma Termini. If that's the case there will be no seat assignments on the Regionale but there will be on the Intercity train.
• Regionale trains are always the same price so using advance purchase tickets won't save you any money.
• You'll need to know the specific names of the stations you'll be using in Italian when buying tickets.

Although Trainline has its place and is a better source for tickets than RailEurope, the first place to check is always the National Rail company (Bahn, Swiss Rail, French Rail, etc). If they sell the tickets you want, and you can navigate their website, they are always (at least with the Bahn) going to have better prices than Trainline's prices.

Generally, the Bahn sells tickets for trains running entirely within Germany and for train connections with one end in Germany and the other outside. For most countries outside Germany, Bahn tickets end with the first change of trains in the other country. However, it seems like there is an agreement with Switzerland, whereby you can purchase tickets from the Bahn with multiple train changes in Switzerland. For instance, you can puchase a ticket from the Bahn website for a connection from Munich to Mürren via Zürich, even though you change trains in Zürich, Bern, Spiez, Interlaken, Lauterbrunnen, and Grütschalp, and it includes a leg by cable car. Trainline does not sell a ticket all the way to Mürren. It does sell it just to Lauterbrunnen, but then for a little more than the Bahn price all the way to Mürren.

advice on getting from Brugge, Belgium to paris non stop by train...thank you

Not possible, until somebody does a Little Rock to Barcelona nonstop.

advice on getting from Brugge, Belgium to paris non stop by train

Not possible, trains don't work like that. That is like saying I want to get from a small town in Kansas to Chicago, but I don't want to turn at any road junctions, I need the same road all the way, and I don't want to stop at any traffic lights either - not possible.

Trains, like roads are an interconnecting network. You start on one train (from Brugge), which will make a number of stops,one of which will be Brussels Midi and then it will continue elsewhere. You get off at Brussels Midi change to a High Speed train (Thalys) heading for Paris.

For info on how to do this, how to buy tickets, see here: https://www.seat61.com/international-trains/other-cities/trains-from-Bruges.htm#Bruges_to_Paris