"Briefly, a date nail is a nail with the date stamped in its head. For example, a nail with a "41" is from 1941. They are usually 2 1/2" long, with 1/4" shanks. Date nails were driven into railroad ties, bridge timbers, utility poles, mine props, and other wooden structures for record keeping purposes. ...

Most date nails are steel, though many are copper, aluminum, malleable iron, or brass. Lengths run from a paltry 3/4" up to 3", with shank diameters running from 1/8" up to 5/16". The nail heads can be round, square, diamond, pentagon, as well as other rarer shapes. Over 2,000 different date nails were used by North American railroads which show the year. Add to that the nails which tell wood, treament, and other information, and toss in all date nails used in poles and other timbers, and the total number of different nails from this continent easily exceeds 3,500. ...

When North American railroads began to experiment with treated ties in the second half of the 1800s, it was not known which chemicals, treament methods, or woods were most economical. They needed some method of keeping track of the lives of ties, so like their European counterparts, they decided to mark them. ...

By the late 1800s American railroads settled on the use of date nails. The oldest known North American date nail is a 97 from the Mississippi River & Bonne Terre. It was in 1899 that major railroads began using nails to date ties with nails ...

By the 1920s nail use was the norm. It peaked in the early 1930s with over a hundred different railroads using date nails in 1931. ..."


Source:    Information signs, Twin Bridges Museum, Lyle, Washington, visited September 2015.