Planning to fly to Frankfurt, Germany, and from there go to Berlin, then Paris, then London, and fly back to US. I know that the Rail pass do not cover the Paris-London train, but for the Frankfurt-Berlin and Berlin-Paris would be better to buy the 2 country rail pass or to buy individual tickets?
You have to do your own homework. Go to bahn.de and price the tickets. Compare to the rail pass and decide if one is cheaper than the other. In the old days, a rail pass was a no brainer but now with discounts for advances purchase, additional fees for high speed trains, etc., you have to do the individual pricing. My gut reaction from my experience is that p2p tickets would be cheaper especially if you took advantage of the discounted advance purchase tickets.
No railpass! Frankfurt-Berlin: Book ASAP (up to 92 days allowed) on bahn.de to get a discount fare as low as €49.00. Berlin-Paris: Book ASAP (up to 92 days allowed) on bahn.de to get a discount fare as low as €69.00 on high-speed daytime trains. Or book a discount fare on the direct CNL night train for a bunk in a couchette or a bed in a sleeper. Paris-London: Book ASAP (up to 120 days allowed) on eurostar.com to get a discount fare as low as €50.00. Discount fare tickets are for a specific departure date and time and are non-exchangeable and non-refundable. Be sure you can commit before booking.