I can’t imagine you’ll have a problem making your reservation after you arrive in Paris.  A couple of days in advance should be plenty for a Thursday trip in April.  If it were the Thursday before Easter, it might be a problem, but I don’t think you’ll have any trouble getting a reservation for the 5th.  Check availability at www.sncf.com  If you don’t have a railpass, you could buy the ticket on-line and perhaps take advantage of any specials that might be going on — there are still some 2nd class couchettes available for 75 euros for the ticket and reservation.  If you have a railpass, wait until you get to Paris to buy the reservation.

While the EN221 is the “best“train on this route, there are other overnight trains but they have a connection or two and leave from other train stations.  I’d bet you could get on one of the trains as late as the afternoon of the 5th.  

how do you check when trains a running. I also have an overnight train ride from Nice to Rome that i want to try and get onto…but not sure how i can book this?

Andrighetti:  Why do you want to book this ticket before you get to Europe?  It’s usually significantly cheaper to buy point to point tickets or reservations in Europe.

For schedules, the German rail website lists virtually every train in Europe and is the easiest to use in English:
http://reiseauskunft…

To buy the ticket try the French or Italian rail websites in order to get the price you’d pay at the station:  www.sncf.com for France and www.trenitalia.com for Italy. You can also buy point to point tickets from any of the sites that sell railpasses, however prices for tickets from the railpass websites are usually significantly higher than the prices to buy the tickets in Europe.  If you need to buy the ticket in advance and can’t get it from sncf or trenitalia try www.italiarail.com … While this is an American site it often has pretty good prices for tickets that start or end in Italy.   

Thanks for the advice.  We will book the overnight tickets just as soon as we arive in Paris on the 28th.

Just two more weeks until we fly…
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Kanda, you can buy the reservations at any major train station in France.  There’s a TGV station attached to terminal 2 at CDG airport.  If your flight lands at terminal 2 you can buy the reservations before you leave the airport, or if you take the RER into Paris you can buy the reservations at Gare du Nord.

ok so…i will have a select rail pass……….but when i get to europe how far in advance should i try to get ym tickets….the day that i arrive in the city?

Your select pass is your ticket, so you don’t need to buy tickets.  If you choose to take a train that requires a reservation then you have to buy that reservation in advance.  How far in advance depends on when and where (which country).  Generally, about a day in advance for day trains and about 3 days in advance for overnight trains is ok.   Farther in advance is better for August, overnight trains on Friday or Sunday or routes that are particularly popular with tourists.  However, you can often buy reservations up to about 1 1/2 hours before train time and there are many trains that don’t require reservations.  If the train you want is sold out you can buy a reservation on the next train (probably only an hour later), or you can take a slightly slower (because it makes more stops) train that doesn’t require reservations.  

A good plan is to buy the reservation for the train you plan to take when you leave a city upon arrival in that city — while you’re still in the train station so you don’t have to go back to the train station to buy a reservation.