Dambusters.
It's probably a variant on the dambusters bombers as the barometric altimeters then in use lacked sufficient accuracy.
Two spotlights were mounted, one under the aircraft's nose and the other under the fuselage, so that at the correct height their light beams would converge on the surface of the water.
I have a different experience since arriving in Canada, that locomotive lights have two lights mounted at 12' o'clock (One over the other) & two at the 5 & 7 o'clock positions respectively.
From a distance the lights appear as one, but once the minimum safe distance with respect to the observer (Say 1 minute away), the lights "break off" from each other & become distinct individually visible.
I'm not sure if this observable effect is by design or lucky accident.
Biting the hand that feeds IT
