15 feet seems to be a good guess for the height of a locomotive that can be used on most North American standard gauge railroads. Give or take: different railroads may have slightly different clearance diagrams, which can lead them to specify modifications to the standard design for locomotives they buy. (Also to different options: railroads with high clearances often specified roof-mounted horns, whereas close-clearance lines bought locomotives with horns mounted lower: sometimes on the side o the long hood.)
(Erie-Lackawanna had at least one line -- I ***think*** was the line in Pennsylvania that, in the old days, formed part of a non-PRR route between the New York area and Harrisburgh -- with close clearances. Sp they wanted to save a few inches on height: their SD-45-2 units came with reduced-height housings for the roof-top radiator fans. The New York Central had close clearances than many railroads; the beveled edges of the roof-top aftercooler (?) box on their Alco C-430 units were, I think, dictated by clearance concerns.)
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The 14'3" SW-1001 is an interesting case. It was a modification of the SW-1000, which had the same 15" height as the SW-1500: acceptable for units bought by mainline railroads. By the 1960s, when these models were introduced, few mainline railroads were interested in 1000 hp switchers: a large part of the market for them was for industrial customers, some of whom probably had low clearance diagrams, and others had maintenance facilities that hade access to the machinery on full-height units difficult. So EMD "squished" the design to suit these customers.
