I think we are talking past each other. I was speaking as an engineer, from both a physics and engineering standpoint, and having seen tension and sag studies by other engineers for electric utility transmission and distribution conductors which like railroad catenary messenger, hanger and contact wire systems are hung from insulators from overhead supports. The tension and sag does vary considerably with temperature. As an example, this whitepaper I found online describes among other things the methodology for calculations.
http://home.eng.iastate.edu/~jdm/wind/T ... AndHTS.pdf I am reasonably certain that the original PRR engineers used the best methodology available at the time, which would have included these factors, in the design of the catenary system.