We will be taking an overnight train from Munich to Berlin this May. Is there any advantage to buying the tickets now, on line, or just wait until we get to Munich? We will be staying in Munich for 4 days.
1. If you wait until you arrive, you are likely to find the night train, at least for the most desirable accommodations, sold out. 2. If you book at least three days in advance (max 92 days) on the German Rail (Bahn) website, you can get Sparpreis tickets at substantial discounts. However, these fares are "tiered" and the quantity at each price is limited. The best prices usually sell out well before the three day advance purchase limit. So, if you can commit now, get your tickets online from over here. Another thought, the "direct" CNL night train takes a rather circuitous route via Braunschweig and takes over 10 hours (of course, you will be sleeping, or trying to sleep during most of it, so it's not like you are using sightseeing time). During the day, there are ICEs taking as little as 6 hours. With purchase sufficiently in advance, you can get tickets on these trains, Munich to Berlin, for as little as €49 for two people.
If you wait until you get to Munich, not only will the night train likely be sold out, but you will have to pay full fare - if you can get tickets at all. As Lee said, the maximum advance purchase time on the German Rail site is 92 days. The best Sparpreis fare is €109 inclusive for two people sharing space in a four-person couchette or €169 inclusive for two people having an economy two-bed sleeper to themselves. The standard fares are €328 and €388, respectively.