The oldest steel manual I can find right now is a 1942 USSteel Co book giving the weights of steel products.
Of rails is says they are usually ordered in
standard lengths. Heavy rails, over 60# were in
33' lengths with 10% short lengths varying in even feet down to 25'.Or 39' with 11% short lengths varying in even feet down to 25'. Light
rails, 60# or less were 30' with 10% short lengths varying in even feet down to 20'.
When rails were ordered to a specific length
with no short lengths the tonnage was arrived at
by multiplying the weight of one rail by the
number of rails required. No doubt there was an
additional charge when they were ordered this way.
John didn't say what the shipped length of the rails rolled to 62' was. Were those cut to 30'?
As for the length of the flat cars determining the length of the rails, I don't believe that really had much to do with it. The governing factor on the flat cars was more what was needed to handle the items being shipped. And history tells us that as the mills became more proficient
they made better and longer rails. Makes you wonder what some of the old timers would say if you took them on a 55 to 60 MPH ride in a 150 car freight train running on ribbon rail where the nearly 1300'long lengths have been Thermit welded into one continuous length.
Jim