One benefit to having the product on steam is the user community also. Trainz has a great user community but Steam has done a good job to make it easy for users to create guides etc. and keep them easily organized by topics so everyone can find, comment and contribute to the development. The Trainz Wiki is very good at containing all of the same type of information but I personally find the guides in the Steam Library easier to find the relevant material.
Just my two cents.