>Do they have to string wire at a particular temp to make it all work

No, but it has to be tensioned to the desired amount with a strain gauge using a table to compensate for the temperature which obtains during installation. The counterweights then make up for the wire's expansion and contraction (change in wire length) as the temperature rises and falls.

With welded rail, it's equalized to a specified temperature according to engineering deparment specs, meaning it's pre-laoded with the desired amount of tension so that in hot weather there will be no sun kinks and in cold weather no pull aparts (broken rails). The placement of rail anchors is also crucial. The AT&SF equalized its welded rail to 120 deg F between Stockton and Bakersfield and to 110 degrees between Stockton and Richmond; not sure if the BNSF still uses this standard.

Compenastion and allowance for expansion and contraction is part of everything made of metal, like highway bridges, injector plungers, railroad bridges, etc.