Track and wheel standards
The NMRA Fine:HO standards allow for the use of RP25/88 wheels with a flangeway of 1mm.
The standards also allow RP25/110 wheels to operate successfully.
The full S3.1 and S4.1 standards are available on the NMRA website, but the basic dimensions for Fine:HO are shown in the diagrams below. All values shown are the minimum required. The standards specify both minimum and maximum values allowable.
All dimensions are in millimetres.
Track gauge (G) = 16.5
Check gauge (C) = 15.6
Flangeway (F) = 1.0
Wheel check gauge (K) = 15.6 max.
Wheel width (N) = 2.2
Flange width (T) = 0.6 max.
Flange depth (D) = 0.6 max.
Below is a one page PDF showing excerpts from the NMRA Standards.