In the early days of railroads, if I wanted to construct a new track, I had to lay two parallel strings of rail, with the specific gauge for that track kept strictly thorughout the entire length of the line. How was the required precision granted? I think the rail I’ve just laid would slightly glide aside when I tried to fix it with nails.
In the old days, a simple iron bar with “u”-shaped forms at each end to assure the correct spacing of the rails would be all that was neccessary to determine their proper placement. In this age of laser transits and other high-tech folderol I’m sure that they have come up with something else for a track gauge.
Here is an old civil engineering text on track.
http://www.railroadextra.com/tkpage.Html
In particular, there is a section on spiking, which concerns itself with proper techniques to avoid throwing the rail out of guage and so on:
http://www.railroadextra.com/tkwk05.Html
The required tolerances weren’t really super-precise anyway. Look down an old, but still serviceable, railroad track sometime. A lot of them will have visible undulations and kinks in them which apparently aren’t enough to cause problems.
It seems fairly easy to me to have jigs which hold the rails in place until the are secure.