Edit: OMG, I made this seem complicated. It’s really not that big a deal. Trains seldom sell out, so only buy a reservation if the specific train you choose requires it.

Your rail pass is your ticket for almost every train in the countries covered by the pass. The only exceptions are a handful of privately owned scenic rail lines, mostly in Switzerland, and EuroStar, the chunnel train. Some specific trains require a reservation which isn’t covered by the rail pass. If the specific train you choose requires a reservation, then you have to buy it separately. You Can often avoid reservations by taking slightly slower (because they make more stops) regional and local trains, but the express trains between major cities and all true overnight trains require reservations.

I would try to make reservations for day trains about a day in advance, but in January you can probably still get a reservation for a day train up to 1 hour before train time. For overnight trains, I’d try to reserve a couple of days in advance, but unless you’re dealing with a Friday or Sunday night, there will probably still be berths available on departure day. There are trains every hour (sometimes 2 or 3 an hour) between major cities and usually several overnight options (although 1 is usually better in terms of timing and connections) so do not worry too much about reservations.

For your trip, buy the reservation for the overnight train from Paris to Italy at Gare du Nord when you arrive from London. Buy the others as you go. Buy the one for the train you intend to take when you leave town upon arrival, so you don’t have to make an extra trip to the train station. DO NOT buy reservations on-line from the travel agency websites (eurail.com, raileurope.com, rail pass.com). They’ll end up costing double what you’d pay at the station in
Europe.

Look for cheap leisure fares and other specials on Eurostar at www.Eurostar.com Buy these tickets as soon as you are sure of the dates.