I don't know railmodeller as a product, some products you have to renew the license every year or couple of years, but in general that doesn't apply to standalone pc applications (what happens with them is you hit a point where they don't support it any more, can't get support, they don't do updates,etc; that said, with rail modeler programs they don't seem to give much support or updates, SCARM is one of the few examples I have seen of that). The other thing that can happen is the software "loses' the license key, which can mean for example that the license file they use somehow got deleted or corrupted.
If you know what the license key is, usually when a program reverts to 'basic' or 'free mode' there is an option called something like "upgrade to pro" , which if you choose it will ask for the license key. If you downloaded the program originally, you might have an email from them with the key. Often this would be found under the 'help' menu item.
