The tracks we cross every day on our way to work are vintage September 1956 from the Steelton mill weighing in at a hefty 132 pounds per yard. (Click on the photo to see the mark.)
Translation tips for your own fieldwork:
Look for the information embossed on the side of the rail. If the numbers / letters you see don't match with the scheme described here, look on the other side of the rail.
Weight: The first 2 or 3 numbers give the weight of the rail in pounds per linear yard. Here's a chart to further expand the weight into dimensions.
Name and year: You can directly read the name and year. The name can be the steel mill or in some cases the railroad company.
Month: Count the vertical bars following the date to find the month. Nine bars mean September.Yours in railfanning,
Kelly