Didn't you ask any of these questions while at your local hobby shop.

That would be one way to become educated concerning 3-rail O guage and maybe even why even on the prototype,  car sizes vary

my LHS has limited O gauge experts...and I am not n the A list of top spending customers  and I had to get home 5to run a train...and it is true on the prototype car sizes vary but kline and mth together ? comical at best..based on appearance in the boxes

Well First you ought to give up drinking anything stronger than tea that is.
next time your liable to dream you ate the worlds biggest marshmallow only to wake up and find your pillow missing.

This is the only "Scale " and I use that term loosely that has different size engines considered the same scale only because it'll run on O scale 3 rail track.
You have scale engines that are 1:48. Then you have what they call traditional which is smaller MTH calls it rail king, Williams calls it semi scale but they're all slightly smaller engines made to handle tighter curves.

Oh and all this data is easier to take in if your not inebriated 

David

I had not drank anything except a dr pepper  and only 2 beers count your first and your last..all the rest are just borrowed/.....telling me not to drink is like telling a model railroader not to run trains..not ganna happen

So any manufacturer can use one size truck and skimp on the materials, build smaller rolling stock and take the profits to the bank..I guess I will only buy MTH...at least it will appease my eyes as it rolls by

think globally..act criminally local...Serge A Storms

I think every brand has both "traditional" size and "scale" size rolling stock. 

It didn't really happen because of profit. It all started long ago because this stuff is so big. People didn't have the room for large layouts hence  the 12 or 14 inch passenger cars and engines that'll handle 0-27 or 0-31 curves.
Guys that have 0-72 and larger curves can usually run what they want scale or non scale.
Now lately IMO I think your right they want to much for all of it scale or not.
Oh and if you buy MTH Rail King 90 percent of the time your buying semi-scale.

David

Hi, O "Gauge"  means that it runs on O gauge track.  Gauge is the distance between the outside rails.  O "Scale" means that it is 1 to 48 scale.

All 3-rail trains are O "Gauge".  Most are not O "Scale" because O "Scale" equipment is just too big and requires too large a radius for most home layouts.

Joe

Here's a page comparing some of the different sizes of O-gauge rolling stock.

O Gauge Car Sizes