I recall hearing years ago that the normal length of rail increased as the length of flat cars increased. It is easiest to carry lengths that are a little shorter than the deck of the flat cars. But this must not have always been the case; I have read a 1904 Street Railway Journal that said that the Los Angeles Railway, an electric street railway, bought 60' rail lengths.
I have the book
Railroad Construction, Theory and Practice. by Walter Webb and issued as a text book for students in colleges and technical schools. It has an imprint of 1901, but is copyrighted 1899. Mr. Webb states:
229. Length of rails. The standard length of rails with most railroads is 30 feet. In recent years many roads have been trying 45-foot and even 60-foot rails. The argument in favor of longer rails is chiefly that the reduction in track-joints, which are costly to construct and to maintain and are a fruitful source of accidents. Mr. Morrison of the Lehigh Valley R.R. declares that, as a result of extensive experience with 45-foot rails on that road, he finds that they are much less expensive to handle, and that, being so long, they can be laid around short curves without being curved in a machine, as is necessary with the shorter rails. The great objection to longer rails lies in the difficulty in allowing for the expansion, which will require, in the coldest weather, an opening at the joint of nearly 3/4" for a 60-foot rail. The Pennsylvania R.R. and the Norfolk and Western R.R. each have a considerable mileage laid with 60-foot rails.
I guess that that the ease of handling the 30' and 39' lengths overweighed any advantage the longer rail lengths brought.
Brian Norden