The " Madders" are a really nice group , have lots of interesting things going on there. I belong to the group too. Lots of " American " stuff is running in Great Britain. D&RGW and Santa Fe is very popular for modelers there. We on both sides of the pond all share the same interest , wheels going around on rails ! Steam , Electric , or Diesel you will find it there too.
I see folks from the forum here also on the forum there.
I am am a member of G scale mad as well (member 5!) and thought they are mainly fans of the Red Box, there are some of us attempting to show them the way to other RR's. I find it both useful and interesting; come and join us, the membership is worldwide.
G Scale Mad member here also. I find that they are quite fond of the "red box" trains for good reason: most of them have small backyards typical for UK and Europe and as such they use a lot of R1 radius curves. US profile standard gauge trains just don't work as well (or at all in many cases) on tight radius curves. With a few exceptions they typicaly model UK and European railroads (ya think?) and the "red box" company provided a good array of affordable and reliable trains for many years (and hopefully will continue to do so under new mgmt).
Some Madders who have larger yards with larger railroads and wider curves have caught the US profile fever and are just as enthused. Plus there are Madders from down under (AU and NZ)as well. Several active members are active railroad employees in various parts of the UK - from engineers to signalmen - and there is often something new to be learned while reading their discussions. I have learned quite a bit from those gentlemen plus they are a polite but humorous bunch. Humor is not as direct, insulting, or offensive as some seem to prefer over here on this side of the pond.
That being said, however, I don't think ANY large scale forum offers as broad an array of subject matter as this one right here.