[Geof]

.....on another job we have a precision linear rail setup straight within 0.001" per 60" and the next day it was wavy over 0.005". the old timer working with me laughed at the setup. as he figured the sun coming through a skylight would cause the metal to expand relative to the concrete floor holding the rail every foot. as the metal expanded it was longer and the only way it could move longer was to go wavy like a snake. we tried they next day when it was cloudy all day and the rail was straight again....

(To continue the hijack.)
I read about how this 'snakiness' is prevented in welded railtrack. It is installed stretched so temperature changes just alter the stress in the rail. Of course if the temperature goes too high the expansion overcomes the stretch and the raill buckles. Conversely, if it gets too cold the contraction imposes too much stress and the rail breaks. Both of which do happen but not frequently and the failures are normally found before a 280km/h train tries to go whistling by.