When should I make reservations on German trains for late Dec trip?

It's hard to predict exactly. Is there an advantage for you to making the reservations once you get there? If not, you might as well do it now, just to save yourself the time and worry. If you want to be more flexible, you're probably OK waiting until you're in country. Obviously it's Christmas time so a lot of people are traveling via train to visit family and such, but most Germany companies give people a long break over Christmas so the rush of travelers is more spread out. It shouldn't matter where you buy the reservations provided the station has a DeutscheBahn ticket machine, or a travel center "Reisenzentrum" (which it looks like the Zoologischer Garten Bahnhof does indeed have both) As for eating, it's up to you. It's kind of fun to eat on the train, but the food isn't cheap in the sit-down dining car, and the cafe car is just you know, microwaved stuff basically. I prefer to take a picnic on the train, personally. In Berlin I get burritos and beer to go from Dolores* for the train.

Hi, No advantage or disadvantage as to where you book the reservations you have in mind as long it's at a Reisezentrum in Germany, be it Bahnhof Zoo, Berlin Hbf. (main train station), or Berlin Ostbahnhof. Going to Rothenburg from Berlin means you'll be traveling on a trunk line Berlin-Nürnberg, a main transfer point. I would get the reservations. Aside from the price factor, eating in the Bord Restaurant on the ICE is a nice experience, provided you can get in. I've always found it full.

The main IC/ICE lines will be busy over the Christmas vacation time. We travelled during that time and were glad that we had reservations - there were always people standing. I'd make reservations now to ensure that you get seats next to each other.

There is no advantage or disadvantage where you make your reservations. At Bahnhof Zoo, which is merely an S-Bahn stop, there won't be a serviced counter so you'd have to make your reservations at one of the machines. Advantage here: No extra fee. They do charge more if you require a real person to do them for you. Rule of thumb: Make your reservations as soon as you know for sure which train you WILL be taking. If you book late your prefered choice of seats might not be available anymore. But in all fairness, that wouldn't be too much of a problem I suppose. If you're a group you might prefer sitting in a compartment which you'd probably end up occupying all by yourselves. That's nice and convenient. The next best thing is to being seated around a table on the open plan. There aren't that many tables in the carriages/wagons. In the worst case you end up sitting in rows just like on a bus... Still all fine. Food on board German Rail is ok. And it's convenient. If you buy a sandwich at the train station you end up paying a similar price than on board. Plus on board they also serve some well-edible hot dishes. They aren't cheap but they also aren't outrageously overpriced. You've got to kill time on the train anyway, so why not having a relaxed dinner there...

Thank you for all the nice responses! I took your advice and booked the reservations. I had hoped we could have the flexibility of waiting until the last minute to book reservations so we could make a better estimate of how to spend our time in each city, but you convinced me to make the reservations now. I am glad I did because some types of seats were already difficult to get on one train. We plan to walk constantly from morning until late afternoon and then ride the train in the evenings, so it would be convenient to eat on the train. Thanks for the information on "train food." Andreas, thank you so much for mentioning compartments and open areas with tables. It probably sounds silly to someone more familiar with Europe, but I had not thought about the difference between a compartment and open area.