Interrail Passes can only be used by European citizens and permanent residents of Europe. You'll find the full list of participating countries below, plus loads of other useful information. If you're a UK resident, Brexit doesn't affect the Interrail Pass. You can still purchase and use one as normal.
Travel on as many trains as you like on each of your 3 travel days – perfect for visiting 2–4 destinations. You can use your 3 travel days any time within 1 month of your start date. Spread them out or use them back-to-back – it's up to you.
Because some trains are more expensive than others, or historically have received less income from Eurail/Interrail to cover the large amount of passholders making use of those trains (Spain and France mainly). Reservations are usually a fraction of the cost of regular 1st class tickets, with some exceptions.
Eurail passes do not cover local transport like trams or metros, only intercity trains. Rail Europe is another company that offers similar European train passes.
On all trains covered by Interrail/Eurail: No, just as long as you can carry it. Be aware stowing away large suitcases on busy trains can be a bit difficult, but there often are luggage racks. There are some exceptions on “low-cost” trains, that I know: Ouigo (France & Spain) and Avlo (Spain).
Eurail and InterRail are both rail passes, and while they are similar there is one critical difference - who can buy them. Eurail Passes are only for non-European citizens or residents.InterRail Passes are only for European citizens or residents.
The Pass even includes certain buses and ferries to make sure everything is within reach. Some popular trains like the Eurostar, TGV, and AVE are included in the Eurail Pass, but require that you purchase a seat reservation.