Please don't take offense anyone. I don't hate
the NMRA. I like the NMRA. It's just that they have the wrong
sizes for all the different gauges. Z thru O.
  I won't get into how this happened but suffice it to say
that years ago the NMRA adopted a wheel space and a rail
space that are about 2 inches (prototype) different.
 I have been studying rail and gauge for 45 years and here
is what I've found.
1. if your track is 2scale inches wider than your wheel-sets
then your going to have occasional-unexplained derailments.
2. The NMRA track gauge is 2 scale inches wider than the
same gauges wheel set gauge.

  So understand???? if you set all your wheels to the NMRA
gauge and then lay your track to that same NMRA gauge......
your rails will be a scale 2 inches too wide. If you don't believe 
this....you can check it yourself. I didn't make this up. 
  Now it just so happens that on that same gauge....is the
proper width for the rails. On the opposite end of the track gauge
is a gauge called flangeways.  That gauge is absolutly right
on for the wheels.
  So if you set your wheel sets to the wheel gauge on the
NMRA gauge and then set your track (especially through turnouts)
to the "Flangeway" end of that same gauge......a miracle happens
in that they rarely derail.
  So if you wish to build track that is relatively derail proof,
just use the flangeway end and you'll be fine.
 I have more explanation on my web site at:
  http://www.railwayeng.com/rrhints.htm

So please don't be mad.......I'm not slamming the NMRA.......
I'm just telling you the facts.  Obviously you can do what you
want ....it's your track  but.............
   Also. I've found it best to use a Two point track gauge instead of 
a three point. We already have enough play in the track.....so we 
don't need to "widen" on curves.....It's already
wide enough if you use the NMRA absolute minimum.
 Two point gauges will hold your rail at an exact gauge while
you spike it. A three point won't do that.
I hope this doesn't upset anyone, but I felt I needed to share
this information with some of the folks that weren't aware of it.
 I have a reputation for quality turnouts that work....and this is
one of the reasons for that rep.
 Happy railroading.
-Steve Hatch  WTA

p.s.
 Yes you can lay the rail base to base with code 40 or 55 or 70
or 83 if you use the flangeway end as indicated ubove.  The problem
was never the flangeway but rather the gauge.