You can use any standard gauge double track to lay straight segments, and to make proper track spacings center to center.
NG single track used for the curves, will join to any double track.
Later on after you have the SG straights, and the NG curves positioned, you can delete each double track, one by one, and replace it with single track.
Double track has whoop-dee-doos when making starights and curves. You will find that double track is problematic, and is best only used for track spacing on straightaways, and is best replaced on the finished route once all the curves and gradients are installed.
Make sure to straighten your straight tracks.
And a curve always has two straight tracks on either side.
An "S" curves has a short segment of straight track betweeen the two oposing curves.
As do switches, they have a short segment of straight track on the lead in.
Double track can not be straightened, so it makes whoop-dee-doos in curves.
