There will be some free updates to Diesel Railcar Simulator, as it's still "early access". The next major version, "Version 2", will be called something different and will have more content and features. Version 2 needs to be bought separately when it's published. Are you thinking of Train Simulator 20xx that gets an update every year? It's probably not going to be that way in this sim because there's no year in the title :)
Are you selling early access game and then expecting people to buy the second version of the same early access game?
Not exactly... Version 1, which is called "Diesel Railcar Simulator" is "early access" in the sense that I'm still going to release free updates (v1.x) to it which improve sounds, graphics and add a bit of content. Version 2 will be called something else since it's not only about BR diesel railcars but a more generic train sim. Version 2 will have a new name, much more content and features as listed in the future plans page. It will be sold separately because the new features will take so much effort and money (for outsourcing etc.) to develop that it wouldn't be sustainable otherwise. You can't expect to pay $15 and get several years of development and content for free. Compare to the other train sim products and their DLC.