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Let's take a look at these. Which are the actual definitions for these gauges?
64 mm (2+1⁄2 in) -- ca 1891, Märklin
2 in (50.8 mm) -- 1909: now using the "2" in "2 Gauge" (undisputed here)
2+1⁄2 in (64 mm) -- after 1909: "3 gauge" in UK
63mm -- ?
63.5mm -- ?
First we should keep in mind that no scale factors are involved. Back then, in the "# Gauge" era, model gauges were defined by their size.
a. "64mm" defined in 1891 by Märklin, metric.
b. Article text: After the change [of definition for "2 gauge"] to 2 in (50.8 mm) in 1909, 63mm/64 mm (2+1⁄2 in) gauge was standardised as 3 gauge.
c. "In the UK, No 3 gauge was [defined a] 2 1⁄2 in (63.5 mm)".
d. "63mm" was used earlier on this page.
So "64 mm" was originally defined in metric (Marklin, 1891). In 1909 2 Gauge was introduced the mean "two inch" for another gauge (50.8 mm) (imperial). The size did not change, but the model scale name "2" was moved to mean another size. After 1909, those 64mm gauge rails were still laying there. All clear to me. Then in UK "3 gauge" was introduced to be "2.5 inch" (imperial). The steps are clear, but I cannot reach the one sourced mentioned (print). Rail gauge "63mm" is not used at all (it weas onin this article, but I removed it because it had no background at all).
None of these three steps (1981, 1909, UK 3 gauge) did mention a "63.5mm" definition (in metric).
I conclude & propose:
A. Nowhere "63.5 mm" (metric) is defined as a gauge. It is a conversion outcome.
I propose to remove gauge definition "63.5" completely.
B. The 2+1⁄2 inch definition in UK is most likely meant to be the same gauge as the existing 64mm gauge. The rounding would be off 0.5mm or 1.28% (calculation: 2.5 inches (63.5000 mm)*). This would be an acceptable difference in re-defining and building precision (in other words, stock would be interchangable within mechanical precision).
I propose to declare "64mm" and "2+1⁄2 inch" as being the same.
If OK then A, B, C will be implemented in {{RailGauge}} (talk)
-DePiep (talk) 05:39, 15 May 2014 (UTC) Added: 63mm and conclusion C -DePiep (talk) 06:14, 15 May 2014 (UTC)[reply]
The authority states: 64 mm = 2+1⁄2 inch (two-way). -DePiep (talk) 06:59, 15 May 2014 (UTC)[reply]
NMRA (Tenessee): I could not find a mentioning of anything 2.5in/64mm in [1] and [2]. (quite plausible, I see NMRA mainly works from scale not from gauge size definition; "64mm" is older).-DePiep (talk) 08:45, 15 May 2014 (UTC)[reply]
