Nice, France
There are no direct trains from Nice to Saint Tropez. Rail services from Nice only run as far as Saint Raphael, and there is no train link from there to Saint Tropez.
Therefore, to get from Nice to Saint Tropez by train, you should start by taking a train from Nice to Saint Raphael, which is 40 kilometers (25 miles) away from Saint Tropez. You can find more information about this service here.
Once in Saint Raphael, you can take a bus, ferry, or taxi the rest of the way. You can also rent a car and drive to Saint Tropez. Buses are operated by Varlib, and ferries by Les Bateaux Saint Raphael (ferries only run from early April to early November). Taxis are operated by many different companies, as is car rental. Make sure to shop around online for good deals if you decide to rent a car.
Alternatively, you can take a ferry, bus, taxi, or car from Nice to Saint Tropez. You can find out more about your other options for transportation from Nice here.