Who knows, really? Different games do these roadmaps in different ways- could be a lot of detail over a shorter timeframe (say, a year), or a broader multi-year look at long term goals with fewer specifics. We'll have to wait and see- since they're only putting it out a week before TSW2 releases maybe the former is a little more likely- maybe they wanna wait to see what they really have nailed down at that point
I think it will be a mixture of the two you have said. I’m pretty sure they already mentioned in the Q&A stream that there would be routes and locos on the roadmap that they were confident would be completed (correct me if I’m wrong) but also on the FAQ on the website, they spoke as if they will include new features they will bring to the game such as multiplayer being ‘on the future road map’