Tell-tale (bridges)

A tell-tale, also known as a bridge warning, is a series of ropes suspended over railway tracks to give warning to the driver that the train is approaching a low-clearance obstacle, such as a tunnel or a bridge.[1] In the US, a standard tell-tale design had ropes on 7.5 cm (3.0 in) centers for a width of 2.5 m (8 ft 2 in) over the track, the bottom of the ropes 15 cm (5.9 in) lower than the height of the obstruction, and placed at least 30 m (98 ft) before the obstruction.[2]

A tell-tale warning of a low clearance over a road

Tell-tales are also used to warn trucks and other tall vehicles of low-clearance bridges on roads and highways. In this context, chains are used instead of ropes, and it is frequently the sound of the chains knocking against the truck that alerts the driver of trouble.[3]

References

https://www.google.com/patents/US1467156
Bridge Warning, Maintenance of Way Cyclopedia, Simmons Boardman and Co., 1921; page 246. Note: the equivalence of tell-tale and bridge warning is stated on page 278.
Lyle Muller, Even chains can't help on Iowa Avenue, Cedar Rapids Gazette, May 4, 1997.

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[1] ttps://www.google.com/patents/US1467156

[2] Bridge Warning, Maintenance of Way Cyclopedia, Simmons Boardman and Co., 1921; page 246. Note: the equivalence of tell-tale and bridge warning is stated on page 278.

[3] Lyle Muller, Even chains can't help on Iowa Avenue, Cedar Rapids Gazette, May 4, 1997.

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