The NASG/NMRA track & wheel dimensions were developed to work together as a set
for optimal operation. These dimensions also allowed for some tolerance
(usually only a few thousandths of an inch) to provide some room for
construction variations. The original wheel tread width dimension is a critical
one that allows the wheel to cross over the gaps at the frog without dropping,
and also will require that the track gauge be more accurate. Theoretically, if
your track and turnouts are made to current NASG standards, optimal operation
will be with the use of NASG wheel profiles, and operation with code 88 wheels
will be less reliable. In other words, for optimal operation, if you change the
wheel tread dimension, you should also change some of the track/frog dimensions
too. This also means that ALL of your wheels need to have the same profile
(locos included), whichever dimension you decide upon. Trying to run both sized
wheels on the same track means that operation with one (or both) will be a
compromise. The only commercial turnouts I have seen (in S or HO scales) that
are truly NASG/NMRA are those built using Fast Tracks fixtures.
Best of luck,
Bill
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