Flying water is right, as only about half to a third of the water actually got into the tender. The rest was splashed out of the track pan onto the ground. Someone else may know more about it, but track pans were as much as 3 miles long and had to be located where the track was FLAT for the entire length. Where in Colorado can you find 3 miles of flat ground?. There was also minimum and maximum speeds at which the water could be scooped up - too fast and all the water got splashed out on the ground; too slow and the water wouldn't make it up the scoop and into the tender. If I remember right, the limits were somewhere between 40MPH and 60MPH - way too fast for the NG. Track pans were a high maintenance item and hard to keep working in the winter because of the water freezing. Also in the winter, it was dangerous to work around the track pans because everything would be coated with ice from repeated splashings.
I've heard all kinds of stories about what happens when things go wrong. Everything from the scoop not being picked up in time and knocking out the end of the pan or ripping the scoop off the tender (and sometimes causing a derailment) to the scoop accidently dropping down and scooping up unintended things, like crossing planks (also causing derailments).
In short, track pans are for high-speed water level (literally) routes where getting there on time is important - the NG just isn't in that big a hurry.
Thomas E. Scott Jr., P.E.