Some Southern Pacific (Western Lines) date nails. The nails in the photo run from 13 (1913) up to 42 (1942), They measure 2 1/2" long, and about 3/16" in diameter.

A Southern Wood Piedmont 17. SWP is a tie treating company in
the south which has supplied ties to many railroads. Ties used at
switches come in a variety of lengths, and to make installation easier,
the ties are labeled with these nails which indicate length. The
nail in the photo was driven in the end of a 17 foot tie. SWP switch
nail have ben found in ties on the Florida East Coast, Interstate, Louisville
& Nashville, Seaboard Air Line, and Southern. The nails were
probably first used in the 1970's, and thy are still in use today.
They serve no purpose after the ties are laid.
Note the "T" on the shank. That indicates
that the nail was manufactured by the Tennessee Coal & Iron division
of U. S. Steel.
1 x 1/4 rnd R stl (19) 17

This mystery nail measures 1 1/2" long with a 1/4" shank. Note
the chisel point.. By its shank markings it is clearly from Canada.
It was made probably in the 1940's or 1950's, since there are date nails
with identical shanks & number style from that time. My guess
is that it was found in a utility pole. No one knows what the T stands
for. Al Byers traded for this nail from someone who has not idea
what it is. Charles Sebesta has other nails in the same series:
a C, I, X, and Y. He, too, traded for them.
Often nails turn up at flea markets, railroadiana
shows, or estate auctions. Usually such nails are easy to identify
because they just about all came from the same railroad. But once
in a while we get a nail like this one whose user is unknown.
1 1/2 x 1/4 rnd R cp stl (38) T (cp = chisel point)
The vast majority of date nails are made from wire, but these are cut nails, like old square nails. They measure 2 1/2" long and have raised numbers. The nails in the photo are from the Toledo, Peoria & Western. The Norfolk & Western was the biggest user of cut nails---from 1932 to 1953.
2 1/2 x 1/4 cut R stl (07) 36-41
Here is a round raised 36 from the Southern Pacific of Mexico. When I published my book I lacked photos for most nails in this series (see page 72 of Volume III). The head was mashed to an odd shape when this nail was made. Most have round heads.
2 x 1/4- rnd R
stl ( ) 36
(1/4- means slightly under 1/4". The manufacturer is unknown, thus
the "( )".)