What do you do to improve a range of a steam locomotive? It's not the coal, you probably have plenty, it's the water, the thing giving you, well, the steam. Normally, you just stop every hundred kilometers or so (obviously depends and varies, but i didn't pull the number out of my ass, it's to give you a rough idea) and refill the tender, you may still see preserved towers with little spigots or similar equipment around.
Except in the UK, where they added little basins between the rails in a lot of places and the train would scoop the water from them without stopping (it'd obviously have to run at certain slightly higher speed for it to work, it wasn't a pump). This may seem odd, but in fact this was used all over the country for the entire 20th century until the end of the steam in the 60's.
Old people may still remember getting drenched by the water splashing all around, including the first few carriages. You can hear about it in old train travel newsreels.