One small word of warning - most of the main stations are huge and can take 5-10 minutes to walk to a platform, examples being Rome, Paris and London Kings Cross.
Although there are only a few places such as Eurostar where an airline type security is present, I would take the “turn up with a minute to spare” advice with a pinch of salt. 5 to 10 minutes early is realistic to find your platform and then board and find your seat. In the UK they advise that doors will be closed 1 or 2 minutes before departure.
I live in Leeds and I assure you I need at least 10 minutes to get to the extreme platforms, not to mention the few minutes it takes to get through the automatic ticket barriers.
Stay relaxed and arrive in good time, If early find the bar.