In the heyday of steam one would see water towers in the yard so the engines could fill up before departing and all along the main line (every 60-80 miles) to replenish the supply. I believe the engines loaded up in the yard (as they would coal or oil) before hooking up to a consist. On the main line they would naturally pull up to the tank load water and keep going. So if you face your tank to the yard spur you are ok moving a single locomotive under it. You can also position the tower anywhere you want on the main line. If you have a small rural station somewhere you can place the tower a little before the station.