I am trying to tell if you are asking about rail length or rail weight or both.
As already noted both of these depend upon the era. And in some cases it depended upon the use or the purchaser.
There was a relationship between rail length and flat car length. But a purchase might include shorter lengths because some rail lengths were shorter from the rolling mill. I have seen a list of rails lengths in each ship load of rails obtained by the Carson & Colorado when it was being built -- every load had shorter lengths. The purchase was made by weight.
Not a logging railroad, the Carson & Colorado was built with 35# rail in the early 1880s. Later portions on the steeper grades and on curves was replaced by the SP with 60# rail. 35# rail from the 1880s was in use to the end in 1960. Some of the first purchases of rail by the C&C were actually used (or excess) light rail from the Oregon Railway & Navigation Co. The C&C traded by weight new 60# rail for this 35# rail in Oregon.
Rail length. It could depend on the purchaser. I saw a reprint of a c1900 trade press article about the Los Angeles [streetcar] Railway. It was reported to be buying 60' long lengths of rail. I guess that could be transported on a pair of flat cars -- after all, think how the long lengths (like 1000' for longer) are transported today.
Brian Norden