Hi Manny ... a lot of the German manufacturers exported to the States both before and after the war , Bing for example produced many models specifically for the US market and along with manufacturers like Dorfan set up US based business's to market their trains ( Fandor being the US side of Kraus/Dorfan) Hornby even had a crack at the US market but was brought down by the Great Depression and they closed their plant in the States after only a few short years 1927-30
Postwar contributions were often low cost sets destined for the chain store market , which Japan and Asia soon took over with cheap and cheerful at much lower costs
After the war ( WWII) many European sets were sent stateside from factories engaged in reparation and rebuilding the industry , but sadly the O Gauge market was not to be so long lived in Europe and the rest of the world , the war and technology was killing them off as the world in general turned on to more HO based home sets .
I would think by far the greatest numbers of quality german/european sets landed in America in the "between wars" period 1920-1940 and with mainly lesser tier factories like Bub/Beckh/Distler after the war ...
No doubt there were trickles and imports at all other sides of the above timelines , because lets face it USA = Expendible Income , you had money and werent afraid to spend it lol !
If you have a few days or weeks you want to spend on youtube then you would maybe pay Fred's Youtube channel a visit 
https://www.youtube.com/user/sncf231e/videos
Just the clockworks
https://www.youtube.com/user/s...arch?query=clockwork
and just one of many to whet your whistle 
