Re: rail size

Rail size really doesn't have anything to do with gauge, but axle loading. I think it was a Baldwin catalog reprint that recommended a minimum of 1 pound per rail for each 300 pounds of WHEEL load.

That makes the approximate axle loading:

35# rail = 21,000 punds per AXLE (35 x 300 x 2) or 42 tons on four axles
40# = 24,000# or 48 tons on four axles
45# = 27,000# or 54 tons on four axles
60# = 36,000# or 72 tons on four axles
75# = 45,000# or 90 tons on four axles

I think the original transcontinental railroad was built with something like 35# or 40#. When cars (ng or sg) are only 10 or 20 ton capacity, the small rail was enough. The limiting factor in steam and early days were the locomotives, since they weighed far more than the cars. If you wanted bigger trains, you needed bigger locomotives, which meant you had to invest in heavier rail and bridges.

As steel replaced iron, and controlledcooled rail became available, the railroads would replace the lighter rail for heavier on the main tracks. While the above list is a guide, it doesn't always hold true. 60# rail shouldn't see anything bigger than a GE 70 ton on it, which when cars only carried 50 tons it worked fine. But in 1992 I spent the whole year running trains up to 8,000 tons at 131 tons each over 80, 72 and two 10 mile stretches of 60# rail. The line had been built 1900-1915 using relay rail from the 1890s off the mainline.

The most popular rail size in the west seems to now be 132, 136 and 141#, for cars weighing 286,000 or 143 tons gross. (Which using the 300# per wheel formula works out to needing 120# minimum.) The only difference between these rail sizes is the thickness of the head of the rail, allowing more wear for curves or rail grinding. There are many railroads struggling to keep enough ties under 90# rail to handle today's weights, in many cases limiting cars to 263,000#.

On narrow gauge lines, the original rail size was generally what they could afford; replaced by heavier rail due to either heavier locomotives or used rail being available at the right price. I think USG at Plaster City is now all 90# for 50 ton capacity cars (~75 ton gross limit).