The Straight Dope (Cecil Adams, 1986) writes:

When the first primitive railroad signaling devices were developed in the 1830s and 1840s, red meant “stop,” green meant “caution,” and clear (i.e., white) meant “go.” This system had several defects. One obvious problem was the fact that the white signal could easily be confused with an ordinary white light. What was worse, however, was the fact that the system wasn’t fail-safe. This was tragically demonstrated sometime around 1914. The red lens supposedly fell out of a signal so that it erroneously showed a white indication. This caused a train to sail through the “stop” signal, resulting in a disastrous crash. The railroads subsequently decided to drop white and make green “go” [...]

This story is repeated in a few other places on the Web; e.g. TodayIFoundOut cites the Straight Dope story and embellishes it with an exact date and a second train:

The choice of a white light for go turned out to cause a lot of problems. For instance, in an incident in 1914 a red lens fell out of its holder leaving the white light behind it exposed. This ended with a train running a “stop” signal and crashing into another train.

GreenLightOptics omits the details, shifts the timeline as far back as the "mid-1800s," implies that there might have been multiple such accidents, and incidentally confuses the engineer with the conductor:

The mid-1800s in England saw many accidents on the railways. Because of this, a standardized color system was put into place. The rails would now use red to indicate danger, white to indicate safety, and green to indicate “proceed with caution” [...] To indicate danger or proceed with caution, a colored lens was placed in front of the light or lamp. But, those lenses could shift and fall out of place, leaving a white light blazing. Conductors would think that meant go, and would continue onward causing an accident. It is said this is when railways chose to adopt green for go [...]

This is a beautiful story, but it shows all the classic signs of an urban legend that gets vaguer and vaguer the farther back you trace it. I eyeballed Wikipedia's List of rail accidents (1910–1919) in vain — but there were so many recorded railway accidents that I'm sure I could miss the right one even if I searched every decade back to the "mid-1800s."

Did such a railroad accident ever actually occur? If so, when and where?

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    "embellishes it with...a second train" -- I think this was already strongly implied, since the point of the signals was to maintain spacing, i.e., to prevent a train from proceeding if another train was too close ahead. Crashing into a slower train (not seeing it in time to stop) was the main problem the signals were addressing. Red signals for other reasons (such as damaged track or a raised drawbridge) would be rare in comparison. – nanoman Jul 6 at 1:50

A lot of confusion here, but a kernel of truth. The book Railroad Signaling (2003, pp. 47-49) gives a fairly detailed account of how the color coding of signals evolved over time. It mentions that the use of clear lights did fall out of favor in Britain due to the Abbots Ripton rail accident in early 1876. The signal failure wasn't the only issue, and it was swing arm stuck in the wrong position, not a missing lens.

In the United States well after 1914, multiple systems remained in use, including some with clear lights (see King 1921, esp. p. 11). The first use of the modern configuration in the United States dates to c. 1900 and it was supported by research on color-blindness. At some point the Interstate Commerce Commission banned the use of white light as a signal for "clear", but I'm unable to find out when that was and whether any specific accident may have lead to it.

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    Page numbers in a nearly 400 page book would be really useful - almost to the point of absolute necessity. – Pieter Geerkens Jul 5 at 20:34
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    @PieterGeerkens Thanks, I've made an edit indicating that p. 11 is especially relevant. – Brian Z Jul 5 at 21:00
  • Clear means GO in many streetcar/tram systems across the world (Madison, Wisconsin, USA uses it for certain bus routes). The difference is that the GO signal is rectangular, not round. It was mentioned in Madison that they use the clear light because it won't be confused with motorist signals (Red/yellow/green). I've seen the same system in use in Freiburg, Germany for their tram system. – Jurp Jul 5 at 22:08
  • @Jurp With the Dutch bussystem, clear (white) is still used for go. However, the placement of the lights relative to each other is also a defining factor, making it possible for colourblind people to still determine the 'state' of the light. Is this not so in other still-used systems? – Mast Jul 6 at 12:26
  • They've just installed some dedicated bus lanes in Indianapolis. All the bus lane signals are clear/white. They're differentiated as a vertical rectangle vs a horizontal rectangle. Again, to differentiate them from the red/yellow/green lights for the rest of the traffic. – FreeMan Jul 6 at 13:31

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