The only thing standard about HO (as practiced with North American prototypes) is the scale ratio, 3.5mm = 1 foot, also stated as 1/87.1
The AAR specifies a series of 'plates,' which actually give the dimensions of the hole in a wall that the locomotive (or car) has to be able to pass through. The NMRA gauge is the right shape, but a compromise size - good enough for most purposes, but not for some things (like double stack container cars) that have come on the scene since it was developed half a century or so ago.
Stix showed how steam locomotives could vary in size. Diesels have the same sort of size range, from industrial switchers that resemble a truck cab and engine on four wheels, out to the UP monster that ran on two eight-wheel trucks.
Passenger cars weren't as diverse, but they were (and are) long, and thus require wider radius curves than small freight cars. Freight cars, on the other hand, can be as short as 24 feet over couplers (ore jimmies in taconite service in Minnesota) or as long as 89 feet over couplers (those humongous hi-cube box cars.) Special service cars can be even longer, wider and higher, culminating in the Schnabel cars which are designed to carry immensely heavy (and immensely oversize) machinery. Even empty, those cars are longer, wider and higher than anything else on the railroad.
WHEN you model also makes a difference. During WWII and just after (the 'transition era') the dominant box car was the forty foot AAR standard design. Today, all those 40 footers are history and box cars shorter than 50 feet are almost unknown. Likewise, huge steam locomotives were replaced by multiple-unit assemblages of comparatively short first generation diesels - which then became longer with each new 'generation.'
I won't even mention the plethora of gauges other than standard. I once printed out a list of track gauges from a website whose link no longer works. Common carrier railroads have been built in gauges from 15 inches to 7 feet 1/2 inch - and there are seven pages of sizes and users between!
Chuck (Modeling Central Japan in September, 1964 - in 1:80 scale, aka HOj)