Reservation for Copenhagen to Luneburg

Try to take a direct train from Copenhagen to Hamburg, if it suits your timings - that way the price charged is only 30 kr.

Counterintuitively enough, the DB website isn’t db.de, but bahn.com. It can book some trains that cross borders, but probably not most international trains. Even where it cannot be used as a booking tool however, it is still usually the most accurate way of finding out timetable information for trains throughout Europe.

As Seewulf points out, almost all trains in Germany do not require an obligatory seat reservation, but if you want one you can click “Seat only (no ticket) at the bottom-left corner of the booking box on the main screen. It’s €4.50 per train in 2nd class and €5.90 in 1st. Trains from Germany to the Netherlands, Belgium and Luxembourg also do not require reservation.

However, to get to Paris in a day you probably will need to pay the reservation fee on your final train into France, because the TER (local train) alternatives are likely to be too slow. There are two good options for this, with different price points.

First, go from Luneburg to Cologne (you’ll need to change once, either in Hanover or Hamburg).

Then you can take a direct Thalys to Paris, at a cost of €30 (either with the Interrail booking app, which adds a €2 surcharge: https://www.interrail.eu/en/book-reservations , or at b-europe.com, which adds €4 for a printable e-ticket). This option makes much more sense if you have a 1st class pass, as *theoretically*, you’ll now get lounge access and a rather stylish meal with wine during your journey, provided the Premium class passholder fare quota isn’t sold out by the time you book. But always check this is indeed the case before you buy!

OR, you can board a CFL train (no reservation required) direct to Luxembourg, and change there to a direct TGV to Paris which only costs €6 to reserve, the cheapest of all the TGVs. This train can be booked either through the Interrail app, or in person at a main station in Europe. The Cologne-Luxembourg train takes in both the lower reaches of the Rhine valley line (up to Koblenz) and the entirety of the Moselle valley line; both are extremely scenic, and the train itself helps with the viewing by being a double-decker!

EDIT: I spent so long writing this that I didn’t notice someone else had already answered the question - I’m glad they did because I completely forgot about the Frankfurt/Mannheim/Stuttgart/Karlsruhe TGVs/ICEs!