Lay it straight!

[Herc Driver]

OK - all you who know the secret of laying flex track laser straight...what's the secret? I've tried straight edges made from wood (which wasn't accurate enough) and metal tubing (which was way too long and not a piece I can cut to length). I'm just trying to get the yard tracks and mainline tracks that parallel them as straight as possible. I know in prototypical yards the tracks aren't necessarily the straightest in the world, but the mainline tracks always seem to be perfectly straight almost anywhere you go. When I use the Mark-1 Eyeball test, there's just a hint of wave to the tracks. Short of using a laser, what's the secret?