Steam Locomotive Question

[nobullchitbids]

I recall at least one of the drawings in MR Cyclopedia, vol. 1, showed a locomotive with such a "smoke light." The stated purpose was to give some indication of the efficiency of the firing. Yes, a coal burner will smoke, especially starting, when the bug dust is blown from the tubes, but once under way, the objective was to NOT make smoke -- no smoke meant more complete combustion and therefore less waste of fuel.

Most of what one sees escaping from a steam stack is -- steam. Ideally, there is no smoke at all.

[nanaimo73]

I read somewhere that the St. Louis-San Francisco Railway placed a light on top of their locomotives ahead of the stack so the fireman could see the smoke at night to better fire the locomotive. This seems like a good way to save fuel. Did any other railroad do this ?