Date nail

[2] Date nails can vary in size, shape, length, material, and rarity.

An example of a code nail would be the Chicago and North Western Railway "FNB" nail meaning First National Bank, which would be hammered into the ends of some C&NW ties.

Another example is the Milwaukee Road "R08" nail, where the "R" is an unknown treatment or type of wood, and the numbers indicate the year it was installed.

[3] Date nail use has dropped dramatically since the mid-20th century and the advent of more modern maintenance of way equipment.

Ties are no longer marked in this manner in North American practice, and the nails are now sought after by railroadiana collectors.