As stated, the length of rail was originally based on the common length of rail cars. A 39 foot rail would fit in a 40' gon or on a 40' flat. Also remember that there was limited mechanization in the early era of railroading, so there was an advantage to keeping rails small enough they could be man-handled by a small crew. A 40# x 30' rail weighs 400#, a 60# rail x 39' weighs 780#. A modern 39' x 136# rail weighs 1768#, a 78' rail double that not to mention handling the length.

IIRC rail was originally made by shearing an ingot, then rolling the blooms into rail. Didn't rail have marks on it showing what part of the ingot/bloom it came from as certain rails would be of varying quality? Real early rail that was made from natural iron ore (vs. taconite) has a high scrap value, supposedly for making razor blades. But early rail before controlled cooling became standard is generally avoided or only used in yard tracks.

The steel mills developed continuous bloom casting allowing them to create long single pieces of rail after rolling, but I'm not sure many US mills upgraded to this. UP a few years ago used a field welder to weld the rail in some of the sidings around here. I've seen them either just take the joints off and weld them, or crop the boltholes out, slide the next trail up, and weld them together. Likewise much of the salvaged rail sent to a rail plant will be inspected, cropped and welded into strings.

The advantage of the 78' length for modern bolted rail is that it can be replaced by two 39' sticks if needed without having to cut a rail to fit (or vice versa), and 78' still fits on modern rail cars.