There are NMRA wheel standards and NMRA recommended practices. In HO, the NMRA standard permits .035 inch flanges. The RP25 (Recommended Practice 25) was developed by a few (bless their hearts) modelers who believed that better performance could be had by going to a smaller flange with a proper wheel tread taper and rounded fillet between the tread and flange.
There are several different NMRA RP25 wheels (see RP25 at
http://www.nmra.org/standards/consist.html). Normally used in HO is Code 110. The Code refers to the wheel width in thousandths of inches. NMRA track standards are based upon the Code 110 wheels. Code 64 wheels are very close to exact scale, Proto87. However, Proto87 track standards are different than NMRA to account for the much narrower wheels. Proto87 wheels will generally not run very well through NMRA turnouts, and Code 110 RP25 wheels will not go through Proto87 flangeways and/or guardrails.
NWSL offers Code 88 wheels which do track fairly well through NMRA standard track. These have a better appearance, mostly due to the narrower tire width rather than smaller flange size. Code 88 wheels are most commonly used in HOn3 because the smaller diameter wheels magnify the visual impact of over-scale tire widths. NWSL also offers Code 64 wheels gauged either for Proto87 or NMRA track. The latter have problems with some commercial turnouts. See the NWSL site HO wheelsets brochure for details. Unless you are going to hand lay your turnouts (in which case you choose your standards), the recommendation is to stick with Code 110 or Code 88 wheels (the latter for better appearance, again mainly in tire width). With Code 88 wheels you may be making "adjustments" to some commercial turnouts for derailment-free performance - but you do that already, don't you?
Due to the inherent "slop" between the HO gauge for wheels and track, the prevailing wisdom is to keep the track gauge right at HO minimum throughout the turnout, and adjust the frog and gaurdrails to correct flangeways and check gauge accordingly. Many plastic frogs are not sharp enough for reliable tracking, which forces dependence of the guardrails and a filled in frog flangeway for proper wheel guidance. When this happens, deep flanges are a problem.
Proto87 requires tight track standards. The magnitude of acceptable vertical irregularities that are acceptable is much smaller. Trucks, in general, must be equalized. However, when this is done, the tracking is every bit as flawless as you might expect. See the video on the Proto87 web site for the demo.
The RP25 Code 110 wheels really didn't become universal until the late '60s. US HO manufacturers generally used .035 flanges, and others have discussed European manufacturers - Rivarossi was the predominant import, used by AHM. The interim Rivarossi .039 flanges and the older US .035 flanges will run on Code 83 track, but will often hit the spike heads on Code 70 and smaller track. RP25 wheels do fine until you reach Code 40 track, where the sufficiency of flange clearance depends on the height of the spike heads. That is why many modelers glue or solder Code 40 rail, rather than spiking it.
Hope this is of interest to some.
yours in tracking