When were the original iron rails upgraded to heavier weight steel?

The original rail weight was 56 lbs. per yard on the main line and 52 lbs. per yard on side tracks, on the CPRR. It was purchased by Judah's agent in the East, the first 5,000 tons from the Bay State Iron Co., in 1862. That rolled iron rail was manufactured in 1863. (The rail is sometimes incorrectly described as cast iron, but is actually rolled from a cast ingot which is then forged before rolling.) The 56 pound per yard iron rail was kept in use through about 187l, when C. P. Huntington purchased steel rail manufactured by Terrenoire from Paris, France. The Boston-based ship "Herald of the Morning" that brought the first iron rail from Massachusetts also brought the first Terre Noire steel rail from France. As the engines and trains got heavier, it was necessary to get larger rail to handle the larger trains. The U.S.  more