Massive? Well, start from next to zero and anything would be massive. That Steam blurb sure contains a lot of promises, be interesting to see how it all pans out. Since I've even already built that line with parts all available in Trainz, we'll see how well they do.
What really caught my eye was the line "Add-ons required to unlock the full list of layers." as well as "A selection of static, AI and a handful of playable services will be available for players that own other Train Sim World Add-ons." So even if you part with €39.99, you still don't get the full experience. And even then, how locked in will you be to only do what has been programmed in?