For searches, the larger sites will almost always dominate.
Reddit has a subreddit/forum for practically everything. This results in a very high number of inbound links to Reddit posts, and of course high traffic. In turn Google sees this as a popular and relevant website and it gets higher search result positions.
This forum actually does quite well, I suspect Tim part because it's not just trains. I'm sure Yorkie was saying we have one of the largest, if not the largest, bus forums, despite being called RailForums. All the engagement there will help us in search results.
Google results these days are personalised but even Incognito browsing I find these forums pop up higher than Reddit for a lot of rail questions. There's a lot of high quality content here which just isn't there on /r/uktrains on Reddit, and I don't feel Reddit really encourages detailed discussions in the same ways traditional forums do. Top level comments getting upvotes though, absolutely, and this often means short, snappy, sound bite style comments get made the top comments.