Hi everyone! This will be my first post in English, so I apologize for any mistakes. Now, the important: how to travel by train in Europe.
I'm going to start talking about how to travel in Italy, but the procedure it's pretty much the same all over Europe.
First, buying the tickets. You can buy your tickets on the internet, on the machines or on the box office (biglieteria, in italian). And usually you can use cash (content, in italian) or credit cards. But attention, in Italy some machines accept both and others only accept credit card.
In Italy there are two mayor companies: Trenitalia (which sell tickets to most lines and from different train companies) and Italo Treno, that sells mostly for long distance trains. So, pay attention to what companies is your ticket, as you will need to validate it BEFORE YOU GO ON THE TRAIN.
There are two sizes of tickets from Trenitalia, but both use the same validating machine. So if you have the smaller ticket, just align it to the left.
There are several validating machines all over the stations, especially by the platforms. You need to insert you ticket (biglietto, in italian) completely in the machine, usually align to the left and wait for a moment until the machine stamps the date, hour and place.
If you are on the train and don't have the a validated ticket, which means that 1) you didn't validated you ticket on the machine or 2) you don't have a ticket at all you can receive a fine up to 200 euros!!!
You can see the validation on the top: is the day, hour and place.
On the buses works similarly, with the difference that the validating machine is inside the bus.
In Venice, if you use the vaporetto, you also need to validate BEFORE you enter.
I hope this was helpful!