There are many model railroad fora (the plural of forum) out there, most of which are operated as multi-gauge platforms. Suggest you Google search for "model railroad forum" and just check out the ones that are listed in reply. Some are sponsored by other model railroad magazines, some are sponsored by manufacturers, and some may charge for membership because they choose to avoid "sponsorship."
Of course, some are devoted to just one specific gauge. If that's what you are looking for, add the scale or gauge to your search term (e.g. "HO model railroad forum"). Enjoy the hobby in whatever gauge you choose.