For all train related questions we have a extra forum https://www.flyertalk.com/forum/euro...il-travel-654/ . There you will find a lot of threads about the Eurostar and Thalys trains.

For all Europe rail pass have a look at https://www.eurail.com/.

The answer what is the best depends on what you want to do. If you only want to go to bigger cities, larger towns the train system in Germany, Switzerland and Italy will be perfect. If you want to venture around the countryside on your own time schedule a car is better. Within cities a car will not be necessary and only stay in the hotel parking.