The Global pass: 15 (or 22) days of continuous travel
https://rail.cc/en/eurail-global-pass-15-days
It’s easier because you can travel all the days and in all the countries, but is more expensive.
The Select pass
The Select pass allows you to travel 2, 3, 4, 5 or 8 days in a month, up to a maximum of 4 bordering countries.
This is a valid option, but you can only use the trains in the selected travel days (00:00-23:59 is one day).
You can check all the information and prices in this links:
https://www.google.es/amp/s/www.eurail.com/amp/eurail.en/index.html
https://www.eurail.com/en/help/choosing-pass/what-travel-day
With the Global pass and your travel days in the Select pass, you can use all the trains you want.
In a nutshell
The Global pass is a free way of travel, and you can change your route, add countries…
But it is more expensive.
The Select pass is cheaper, but you need to have planned your route in advance.
Now the decision is in your hands!
P.s: UK is not included in Eurail pass
Enjoy your trip!
