I have been collecting, researching and writing about date nails for a long time now. There is actually a National Association of Collectors of these (GOOGLE T.D.N.C.A.)of which I am a member.
The SP in the west stopped using them in 1942 but the SP Texas lines used them until 1951. The Santa Fe stopped after 1969 as did the Great Northern. The Northern Pacific stopped after 1921 and the Milwaukee Road - famous for using a lot of copper nails because of its affiliation with the Montana copper mines - stopped after 1951 (although they did use a bunch of 1962 Santa Fe nails in Iowa in a test section. The UP stopped using date nails in 1936, but also used a few 1964's found in WY. Most all Class One roads used date nails at some point in their history. Out west the WP did not use them and you will see a line like the Camas Prairie that was formed by a joint venture of the NP and UP in the very early 1900's where both parent lines used nails but none was used on the Camas.
The first known date nail used in the U.S. was the 1897 supposedly used by the Mississippi River & Bonne Terre railway. I have never seen one of these and the earliest date usually found in big collections is the 1899 Great Northern Nail. The use of date nails started as a marker for testing various tie treatments for longevity and was introduced to America by Octave Chanute - of some aviation fame as well! Good nail salesmen seemed to keep the usage of them going for the purpose of marking the year the tie was put into service, regardless of the treatment. A letter or code nail was used by several lines for tests, and timber identification and bridge markings.

Date nails were also used on Canadian, Mexican, New Zealand, French, Belgium, Russian, Japanese and Australian railways. There are a few others I am sure I left out.

More than you ever wanted to know about date nails??
Regards,
Rolland Meyers