How soon to buy train tickets between countries

Others may be different to me, but I'd be looking a few days before travel.

I wouldn't expect any of those to sell out, and there are frequent alternatives.

Are you using the German rail website
https://www.bahn.com/en

If you don't see availability you might be looking too far in advance. I did a search for September 1 2022 and see tickets for sale from Frankfurt to Berlin

Are you using the German rail website

This is the key question. I'll bet you're using Rail Europe, a third-party marketing tool that is using more tools, to get you to buy now. However, this trip is likely to be cheaper bought like an air ticket, non-refundable, non-changeable. It may be hard to get two seats together as a walkup sale, and I recommend avoiding long lines at ticket windows during valuable vacation time! As others noted, this is not ideal for an air arrival. No one who wants to go directly to Berlin should buy a ticket to Frankfurt. It's no longer the gateway to Germany, since the fall of the Soviet Union.

That said, I would suggest a stop in Eisenach, Erfurt or Weimar, if that fits into your plans.

DB train website and app are pretty good. Lots of trains so you will have options. Book a few days or 2-3 weeks before if you want to be safe. The prices may go up a little bit, but that’s the cost for flexibility . The cheapest fares cannot be changed or refunded.

I am flying to Berlin. you must be on Lufthansa too. They have flights direct to Berlin, and connections from Munich and of course Frankfurt - going from NA

Tim, While there is some, limited direct service from North America to Berlin, LH (and UA) have vastly more connectivity through the Star Alliance mega hub that is Frankfurt.

The tickets for the Frankfurt to Berlin ICE train will be available , be it in 1st or 2nd class. Take the train after you arrive in Frankfurt. I've done it a number of times after flying from SFO to FRA but only after a few hours and lunch at Frankfurt Hbf. ..no rush.

Thanks to all who answered. I'm flying UAL from San Francisco to Frankfurt and spending one night, then continuing on to Berlin. I'll wait awhile to look at those train schedules.