Here is a link to a page that gives a bit of information on Railroad Watches.
http://www.pockethorology.org/Railroad/Railroad.htm
I am fortunate enough to own a very few Railroad Pocket Watches. One I carry daily. It was built in in 1926 and it still performs within Railroad Standards. It's a Hamilton 992.
I also own an Elgin Father Time (Eighteen size) that was built in 1899. It also keeps excellent time. BTW, the dates are determined through the serial numbers. There are web sites that publish serial numbers and the years the corresponding watches were made.
Sixteen size is the size most people are familiar with. Eightees size is slightly larger.
I am including a picture of the Father Time.