Last Updated April 12, 2011
A word about the current P:64
standards. The original proposal which was 3/16" AAR proved unworkable
for US prototypes in small scales.
Enter The NMRA Proto Effort. This
effort started with 1/4"AAR and worked till that became P:48. The current Standards use an engineering allowance
called FIT with any scale less than 1:56. As demonstrated by our
original proposal we do not believe this necessary above 00 scale
(1:72). Therefore; We had asked that the spread sheet be modified to
reflect this, the result proved not worth the effort
Recommended Practices are not truly Proto as they contain
compromises to accommodate RP25 wheels. That said they are achievable
with commercial components and using Code 88 wheels from either Des
Plains Hobbies or NWSL a semi-proto layout can be modeled.
Track & Wheel Relationships
To ensure Interchange and reliable performance STANDARDS are designed
so that Track and Wheels constructed within prescribed limits will meet
the following related conditions closely parallel to prototype
1. TRACK GAGE (G in S-3) is the distance between railheads of two Stock
(running) Rails of a length of Track.
a. Straight (tangent) track should be laid as close to the minimum
limit of Track Gage as practicable.
b. Curved track may increase Track Gage as curve sharpness increases,
with such increase applied with care lest the wheel is inadequately
supported by the railhead, and side sway of equipment is exaggerated.
c. Three-point track gages should be constructed so as to fulfill these
2. TRACK CHECK GAGE (C in S-3) is the distance from the flange side of
a Guard Rail to the flange side of the Frog it guards. WHEEL CHECK CAGE
(K in S-4) is the distance from the Back of the flange of one wheel to
the tread-side of the flange of the other wheel of a wheelset.
a. Maximum Wheel Check Gage should not exceed the minimum Track Check
Gage so that Guard rails will protect the Frog. See Fig. 1.
3. SPAN (S in S-3) is the distance between flange
sides of the Guard and Wing Rails at the guarded Frog.
BACK-to-BACK (B in S-4) is the distance between the Backs of the wheel
flanges in a wheelset at railhead height.
a. Maximum Span should be less than the minimum Back-to-Back distance.
4. FLANGEWAY WIDTH (F in S-3) is the distance between flange sides of
the Wing and Frog Rails. TIRE WIDTH (N in S-4) is the distance between
the Back of the wheel flange at railhead height and the outer edge of
the wheel tread.
a. Maximum Flangeway Width at the point of the Frog should be less than
half the minimum Tire Width to ensure the wheel tread riding the Wing
Rail across the intersection of the flangeways until it is supported by
the Frog Point Rail. See Fig. 2.
b. Guard Rail Flangeway Width is limited at its maximum only by Track
Gage and Check Cage (G and C).
5. FLANGE CLEARANCE (H in S-3) is the vertical distance from the
railhead to the highest obstruction below it. FLANGE DEPTH (D in S-4)
is the vertical distance from the root of the flange to its outer edge.
a. Minimum Flange Clearance should not be less than the maximum Flange
6. SWITCH POINT SPREAD (P in S-3) is the distance from the Gage Line of
the closed Point Rail to the outside of the open Point Rail. See Fig 3.
a. Maximum Spread (mechanical) should not exceed the sum of minimum
Back-to-Back plus minimum Flange Width (Bmin + Tmin) to prevent
b. Maximum Spread (electrical) should not exceed the dimension of 6.a.
above minus 0.005" to prevent short circuit between Point and Stock
Rails of opposite polarity where this condition exists.