I think you've missed the big picture. HO is a scale -- that is, a proportion to the size of the real thing. NRMA standards are there to ensure interoperability of equipment from different manufacturers, but it's unrealistic to suggest that everything in 1/87.1 scale (HO) HAS to work on a given layout. Especially in a hobby where there is total freedom to design a track plan to whatever specifications you like. This isn't a toy system where everything sold for the system works with it; it's a scale and a collection of standards to assist a hobbyist.I plan on doing that . what bothers me the most about ho is that you can buy a ho engine but you may not be able to run it on your track because of radius restrictions . its kind of not fun . I have 3 ho engines that are 9 inches long . does that mean 18 in radius not big enough ?
HO track is produced in standard curves down to 15". Using flex track, you can get down to about 9". Our scale equipment is produced with much more swing in the trucks and couplers, and longer distances between cars, to be able to handle these "abrupt" curves. On real railroads, "tight" turns are much broader -- they'd probably scale down to about 33" or so. A lot of hobby products are engineered to be able to use 18" curves because that's what train sets usually come with, but many these days are opting for greater realism and requiring larger radii. Also, grades work the same way. A 1% grade is huge for a real train; we hobbyists routinely use 2-3 times that. On our models, grade itself isn't a factor as much as the transition onto and off of them, where equipment can dig in its nose or become uncoupled or derailed.
The only way to be ABSOLUTELY certain whether your equipment will run is to lay some track and test it. Some locos that say 18" minimum don't do very well with it, and some that recommend broader curves can be coaxed through 18" ones. While a good thumb rule says your curves should be a minimum of 2.5 times the radius of your longest equipment; like all thumb rules, it's meant as a general guide and not a substitute for empirical evidence.
If I had to guess, I would say that if your locos have 2 axle trucks, they will work; 3 axle ones probably not. Since the truck doesn't bend, the cord across the curve can't exceed the distance between the contact patches of the front and back wheels, or they will bind against the rails and cause problems. That's the REAL limit, followed closely by whether there is enough distance between cars to enable them to round the curves without bumping.
If your locos (and trains) are close to making it, but can't quite, there are some tricks that you can use to improve their ability -- all at the price of realism (and only you can say how much of a problem that is).