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This fall, there emerged on the Proto-87 email list an extended discussion regarding some discrepancies between Proto:87, the prototype and standard HO. These discrepancies come in the form of flange size and track gauge.
The less troubling of these two questions is that of flange size: the Proto:87 effective flange dimension is actually smaller than the North American prototype flange. The reason for this lies in the English roots of the specification. Because the specification comes from Britain, the wheel profile is based on a British one and these have slightly smaller flanges and have a different shape than their American cousins. The smaller flange also has a benefit in that wheels with smaller effective flanges can actually negotiate tighter curves than ones with larger flanges.
The slightly more worrying question is that of track gauge. The Model Railway Study Group (MRSG) specification, which is included with this issue of the Journal and shown in Table 1 is based on a scale of 1:87, while NMRA standard HO is based on a scale of 1:87.1. This difference is large enough to cause a thousandth of an inch difference in gauge. Ed McCamey brought this to our attention by mentioning that the NMRA had originally specified a gauge of 0.650 inches, identical to Proto:87, but had later corrected it to 0.649 inches.
Table 1 also shows the MRSG specification multiplied by a factor of 87/87.1 to modify it to Proto:87. Effectively, the difference means a narrowing of the gauge by 0.02 mm (.001 in.) across the board. It does not affect the smaller dimensions such as the flangeway width, or wheel profile, however.
As I see it, we have three choices from here.
First, we could define a new specification that would compromise between the two -- perhaps by constricting the tolerances on Proto:87.1. This would require a whole new round of testing before it would be acceptable to the NMRA.
The second option is to further splinter HO finescale into Proto:87 and Proto:87.1. This would require an amendment to the proposal I published in the NMRA Bulletin last year [Gourley, 1997], and which was reproduced by Paul Dolkos in a recent Model Railroader article [Dolkos, 1998]. The change may further delay acceptance by the NMRA. Currently, we are hoping to see this stamp of approval as a Recommended Practice by the NMRA in 1999. A couple of manufacturers are awaiting leadership from the NMRA before committing their resources to support Proto:87.
Finally we can accept the fact that we may be placing 1:87.1 car bodies atop 1:87 undercarriages in North America, and press forward with the original proposal. We could add a note to the recommended practice calling attention to the difference in scale.
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