I don't do Steam. It is not for sale from N3V?
Afraid not from N3V unsupported since September 2018.
Likely any online store selling it now is a Steam key. If I remember correctly, TS12 will work if you remove it from the Steam folder or run it directly.
Or from N3V's page there is a link to Amazon https://www.trainzportal.com/product/view/trainz_simulator_12 it seems to be a digital download.