basically it changes the look of your train station to something else. it does not give it any new functions, just design. the original starter ones look like this and the upgrade unlocks the option to change them to look like these (the ones on the right) via pwp!
Just to look pretty and it is quite handy if you've got a theme going on in your town as you can get it all to match
But yeah other than looks it doesn't alter anything or have anything special about it