One other reason that the infamous "Blue Peter" slip went out of control is due to her design. It is an older locomotive with a single-valve dome throttle. The throttle controls the flow of steam INTO the superheaters. Once steam is admitted into the superheaters it has an unrestricted path to the cylinders.
This is opposite of the way a multiple-valve front end throttle works. With a front end throttle, the throttle valve is on the OUTPUT side of the superheaters. If the locomotive pulls water, the flow of steam can still be controlled by the throttle because it is controlling the OUTPUT of steam from the superheaters.
If a steam engine with a dome throttle pulls water, the superheaters act as a secondary boiler! Even if the engineer could have gotten the throttle closed, the slip would have continued for a while because once the water got past the throttle, the superheaters flashed it into steam and it flowed - unrestricted and uncontrolled - to the cylinders. This was one of the reasons that the front-end throttle came into use, because this dangerous situation could not develop with a front-end throttle design.
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