date nails were used to date the age of a section of ties and when they were last replaced they would placed every 3-10 ties or so all they were used for was to mark the date in which a a section of ties went in like the first 80 ties were layed in 1925 the next 60 went in in 1939 and so forth. They most ofton dated when ties were replaced. Some railroads had letters on there date nales which told you the type of tree the tie was made from like O for Oak, P for Popler, C for Ceder ect. I used to collect date nails and have enough to cover from 1915-1939 3 nales from 1915 came from the Lehigh Valley the rest came from a line that went threw an unknowen line in Quinebaug, CT. Here are 2 web pages on date nails.