Distance Between Rails?

This is difficult to qualify because it depends on what you are running.

What I do is take a piece of clear rectangular Plexiglas that is the size of your biggest piece. Mount trucks underneath at the approximate locations of that piece. Then attached felt tip pens to the four corners and middles of the plex such that the tips touch the table. Place the plex-car on the track moving the car along the track while scribing the table. The pens create the clearance required for this track.


HO version of plex car

Place the adjacent track and scribe again with the plex-car on this track. When the new scribe lines do not interfere with the former scribe lines you know that clearance has been achieved between each track.

You can also use the plex-car to see how the track is engaging the trucks and mount small levels in the front and back to measure the slopes of your trackage.
I have added vertical plastic to the plex-car to simulate height clearances for tunnels or bridges.

If you are using O-72 and O-81 and have the two tracks parrallel to each other, then the two lines will be on 4.5" center to center. If you want to gain a little more clearence as you go into the curve you will have to start the inside curve a little sooner then the outer. With a 180 degree curve it will wind back up at 4.5" centers, but with a 90 degree curve, you will have to add a little straight halfway through to get back to 4.5 centers if starting the inside curve sooner to gain clearance.