I realise this thread has been bumped this evening from some time ago, but in regards to the original question. I guess one thing this forum has is an attraction to multiple groups rather than specific individuals? What I mean by this is past forums such as The Transport Forum (something like billz1064.proboards.com for those who may remember 'those days'), which was the main rail forum in the early 2000s, primarily catered for enthusiasts and at the time there could be bouts of infighting and the like with little done to break it up (or sometimes deemed done with bias). When 'RailUK' (as was) was launched in the mid-2000s, it became a sort of fresher place to be, set up by likeminded people who wanted out of that environment, with very regular forum meets making it all the more sociable. There were periods I was in London seemingly most weekends, plus longer trips, on forums! That better atmosphere may have helped it attract more people discovering forums at the time and expand. There were still disagreements between members or between members and forum staff, with some attempts at alternatives even being launched (anyone remember 'The Messroom'?), but I think this forum had become so established by then that they were never going to realistically work long term. Moderation also being active meant public arguments don't tend to get left to completely derail topics and see people trying to have discussions give up.
There was a period where there were several train simulation forums which was not going to be sustainable long term and as this place grew and more and more people became members, rather than post the same thing on multiple forums I guess people just got used to what was fast becoming a one stop shop for all their transport discussion needs. Similar with bus forums that have come and gone (or now a shadow of their former selves), which may be helped by the fact there is a good crossover between enthusiasts of both buses and trains. Why do a tour of lots of forums when one single place caters for all your interests?
Once RailChat closed, which was a forum predominantly aimed at/used by staff, I guess this was a logical place for any who weren't already members to turn to. I seem to recall there was a signal box forum that died a death too. Therefore you now had enthusiasts of multiple transport modes, players of simulators, and railway staff all now gravitating towards a single one stop shop, rather than having to visit multiple sites for different specific interests that were ultimately all linked under one umbrella topic.
I had a bit of a break from forums for a few years but when I returned here it had expanded its reach even further with a more active recruitment section, which attracts non-enthusiasts who are seeking advice on a potential new career, and the Fares section with its Dispute subforum, again attracting non-enthusiasts, so yet another audience stream that keeps the forum active beyond just enthusiasts and staff. As the forum has grown to the size it is, I wouldn't be surprised if other non-enthusiasts such as journalists and the like are using it as well.
So part of its success might be that it has successfully attracted users that don't just want to talk about one particular aspect of transport (ie spotting, simulators, modelling, working etc), but all aspects under a single roof.
Or these are just my thoughts anyway!
