If you haven't made your reservations for Eurostar, you should do that ASAP as, at least the end if July, is high season and pass holder seats can sell out weeks and even months in advance. Especially this year with both EURO 2024 in Germany and the Olympics in Paris. 

That's great advice...thank you

Book Eurostar on raileurope.com (desktop website only) to avoid extra booking fees.

Thanks for the reply, is that for the reservations?...then use the interrail pass to board the train.

Nice to Florence will be:

Nice to Ventimiglia by local train

Then the quickest itinerary seems to be Intercity to Milan (€3 reservation fee) + Frecciarossa to Florence (€13). But if you plan to spend time in the Cinque Terre, you could take the route Ventimiglia-Genoa-La Spezia-Florence. The Cinque Terre is spectacular, if a little crowded

Yes, you need the reservation to get through check in for the Eurostar, and Raileurope is the best place to get it.

For any trains which require reservation, you need that + a valid railpass.

There’s a good summary of reservation requirements here. But in general, on your itinerary, you should expect them on any long-distance or high-speed train that touches Italy or France, plus night trains.
https://www.seat61.com/interrail-and-eurail-reservations.htm

That's great. Thank you for the advice ...much appreciated. 

Thanks 

ralderton I wouldn't have known to do that....thanks again.

Just as a summary for your trip:

Reservations aren’t needed in the UK, but you can get one for free if you like - weirdly, from the GWR website.

Eurostar reservations cost €30 to Paris, and €35 back from Amsterdam. TGVs in France are €10 or €20. Intercity is €10, and the sleeper to Nice is €22.

In Italy, Intercity is €3, Freccias are €13. You can buy all these from Raileurope. Just add the railpass. (As mentioned above, only possible on the desktop site, not the mobile!)

No reservation needed for local trains from Milan over the pass in to Switzerland. Just sit in the unreserved coach on the Bernina Express. Nothing needed in Switzerland, Austria or to Budapest (unless you use Regiojet or night trains).

Deutsche Bahn is now requiring reservations on a lot of cross-border trains. This is new this summer, so check on Seat61. They don’t cost much.

Thanks for all this info...it takes time to pull it together...this help me get there quicker....thank you again.

Looks really cool!

Another tip : the Zurich - Vienna night train is quite expensive and sells out fast. A better option is the Bregenz/Feldkirch - Vienna night train → shiny new carriages, very affordable too as it’s subsidised by the Austrian government.

Have a look at https://www.seat61.com/trains-and-routes/nightjet-new-generation.htm

Only downside : earlier arrival in Vienna.

Very good choice to take the Bernina railway. Some trains have open-air coaches if you want something fun (bring jackets!) → https://tickets.rhb.ch/en/pages/open-panoramic-cars

Looking at times you’d have to leave Rome quite early though. Of course the hourly regional trains are really cool too.