The gallons figure is the water in your tender. The percentage figure is how full your boiler is with water. At 30% there's a risk the top of your firebox is not covered by water, so it's going to get too hot and the fusible plug will melt, to stop your boiler exploding. You have to use the injectors to transfer water from your tender to the boiler, as it’s boiled away as steam. If you aren't driving a loco with advanced controls then the simplest way is to click the icon you've circled. Click it again to turn the injectors off.
When you start the Scenario Pause it and go in to the World Editor then Scenario Editor. Double Click on the Loco/Tender and make sure the Water is Full and same with Fuel
This can happen randomly. Using auto fireman might help, in that it knows more numbers. I don't know the exact cause, with the Cab Forward I had it on Pacific Surfliner - despite everything open, I was losing water, possibly the loco is speed capped at 30 mph due to internal physics. I had it with other locos as well, usually just once in a while, so replaying the scenario "helps".
