You can get a Eurail pass but you don't have to, you may as well buy separate tickets. Depending on your trip either option can be the best (although for a long trip a pass should make more sense).

Luggage is stored next to you, on luggage racks, overhead or between seats. You are responsible for it so yes you have access at all time to it.

With Eurail you need to log each train you'd like to take, this can be done at any time before boarding (flexibility). On top of that in some countries you need to add mandatory seat reservations on fast/high-speed trains : mainly Spain, France, Italy and cross-border routes such as Eurostar and TGV/ICE to Germany.

In this case it heavily depends on the route and time of year but some trains need to be booked weeks in advance (resulting in no flexibility). Those reservations are rarely refundable/exchangeable but you can always make a new one if there's still availability.

For more precise advice feel free to post your itinerary.

Have a look at :

https://www.seat61.com/how-to-use-a-eurail-pass.htm

https://www.seat61.com/interrail-and-eurail-reservations.htm

ill be starting in london then train to paris then im flying to zagreb and then bus or train to split and then im flying from split to rome and then train to naples and im not sure yet how to get to sicily whether to fly or if there is train? then if going to ferry to greece adn then ferry everywhere there island to island.

I'm not sure the pass makes a lot of sense then. Make some calculations maybe, even though most trains in March/April won't be on sale yet.

London - Paris : Eurostar has cheap tickets bought well in advance (aka 3 months or so before, as soon as your dates are set). If it's around Easter book before. With Eurail this route has a passholder quota and an expendive seat reservation fee (30€) so the flexibility is none either way.

Zagreb - Split : 15€ train ticket, buy a few days in advance on hzpp.hr. Do you have a specific reason to visit Zagreb ? It's one of the most depressing and boring European cities. I'd either fly to Split directly or visit Slovenia. 

Btw since you seem to have got enough time on hand you could take a train to Stuttgart or Zurich and then board the night train to Ljubljana or Zagreb. Fun experience ! You save a night accommodation and arrive the next morning in the city. If you choose that option then a pass might make more sense ! You only pay accommodation so a couchette or a sleeper (seats are not advised).

Split - Italy : take the overnight ferry to Ancona and then the train to Rome. Better than flying !

Naples - Sicily : there are daytime or night trains to Palermo/Catania. It's a unique route in Europe as the train is loaded on the ferry !

Sicily - Greece : loads of ferries to Patras from Brindisi or Bari. You can make your way there overland from Sicily, even though the journey is quite long. Loads of interesting unexplored parts of southern Italy !

Tickets are cheap in Italy and trains are frequent, at least as far as Naples (buy a few days in advance for better prices). If you opt for Eurail fees are 13€ on Frecciarossa and 3€ on IC trains.

Feel free to ask further questions.

Current schedule Messina - Bari :

- Boat Messina - Villa San Giovanni 08:30 - 08:50 or any other one (plenty)

- R Villa San Giovanni - Reggio di Calabria Centrale 09:44 - 10:02 (the next train makes you miss the IC, unless on a Sunday)

- IC Reggio di Calabria Centrale - Taranto 12:10 - 18:50

- R Taranto - Bari Centrale 19:30 - 20:47

More options, slightly faster but not as hidden gem, via Salerno and Caserta.

You could also decide to spend the night in Reggio and take the morning train (departure 08:00, arrival at 16:57). That way you can take evening ferries to Greece :)

thank you thats very helpful, wahts the best app/website to book ferries?

I'm not an expert on that but I think https://www.anek.gr/en/ covers a lot of routes. Otherwise Direct Ferries could be helpful too.

okay thanks so much