I imagine they would run P-line, then they would need profile so they could "slope stake" the individual footings...
RR builders used many erector and machine alignment methods and a lot of horsepower hours in cutting, moving timbers and driving spikes.
It is a half and half ration between the curves in railroads matching any computed location as plans show and what is measured for the ones built on land. Finding a beginning station point is becoming more difficult with missing accessories that have been updated and modern location of crossings.
The remains of a small gauge track are a few miles away from where it crossed a deep creekbed about 250+ feet long that was -quickly beaver packed and created 50 acres lake. Gone now and covered by a large sandbar of wandering channels.