For me, I figured out that there was something wrong with my Mantua 4-6-0 when, as an elementary school kid, I could tell that it looked kind of large. I noticed the same thing with my athearn passenger cars in junior high...in pictures, the NKP PA-1s were noticeably shorter than the passenger cars...but on my layout they were the same length.
One of the biggest problems with out of scale items isn't that they don't look good...it's that they don't look good with the stuff around them when you either A) check a picture or B) set it next to an accurate model.
Like Kevin said, you can pretty much presume that anything in brass or coming from newer tooling (spectrum, Athearn Genesis, P2K, MTH, BLI, PCM) is fine. Most older models have something off (the Rivarossi berk models have 65" drivers instead of 69"...a compromise due to the original flanges being badly oversized). I have a brass 2-6-0 with several small issues and a brass business car that is 6" scale too tall...quite noticeable next to other On3 cars...because they were built to incorrect plans.
I'll try to explain the scale/gauge issue...since many people are confused by it:
The letter refers to the scale. Scale means that if you multiply the dimensions (length, width, & height) of a model by the "scale", you'll find the dimensions of the real thing. For instance, a real 90' turntable is 90' long, an HO 90' turntable is 12.4" long. I am 6'2" tall; in HO scale, I'd be 0.85" tall. A building in O scale is the same size...whether the layout is O 2-rail, O 3-rail, On2, On30, On3, Onwhatever.
In north america:
F 1:20.3 (Bachmann's large scale 4-4-0)
G 1:22.5 (LGB's original trains)
O 1:48
S 1:64
OO 1:76
HO 1:87.1
TT 1:120
N 1:160
Z 1:220
Gauge is a reference only to the width of the track. Hence real railroads can be described as 56.5"
gauge (standard
gauge), 3'
gauge, 2'
gauge, or 18"
gauge. Those 4 were the "common" gauges in the US in the 20th century. Most of the country used standard gauge...while 2' were most popular in Maine, 3' was most popular in Colorado, and 18" was more for industrial...especially mining.
So, if you want to model 3' gauge railroads in O scale, you want to model On3. If you want to just model standard gauge, the std gauge is implied by saying O scale.
I think we can break up the scale/gauge combination into two groups: natural and convenience. Natural means that if you want to model 64" gauge trains in S scale, it naturally makes sense to use 1" gauge track. Now if you start a company and begin producing 0-6-0s and track, someone might come along whom is a big fan of the
Hecla & Torch Lake and say "Gee, that is pretty close to being correct for modeling 4'1" gauge trains in O scale!" That would be a scale of convenience. A wise person would come along and produce a some equipment that runs on your track. Suddenly many people are modeling On49 (4'1"=49")...the first people whom kitbashed equipment...and the later people whom at least started out by just buying RTR. What I've described is exactly the same situation as what happened with On30 (which is actually On32...just as the example actually scales out to On48)).
Natural scales:
HO, HOn3, On3, On2, Z, G, F, S, Sn3, Sn2, etc...
Convenience scales:
On30, HOn30, Nn3...and sort of O scale...which I'll cover in a bit...
What I'm calling natural scales are where the track actually scales up to the real thing. A scale like F scales up...although the scale ratio...1:20.3...was derived to fit the existing track. Still, while 1:20.3 is not a natural choice for a scale, you would naturally use track that scales out to the right width.
O scale standard gauge is a very odd animal. Both 2 and 3 rail people's track doesn't scale up to 56.5"...instead it scales to 5'. This means that the models must be built slightly too wide for the track...essentially O60 The buildings are still the correct scale...just the track isn't. A finescale movement exists called Proto:48 which uses slightly narrower track so that their trains due run on true O56.5. Additionally, they fix other scale issues such as wheel width...a similar movement exist in HO...and here's a
wonderful example of the wheels on the Proto87 website.
As far as the scale of the track:
Certain parts of the track have certain dimensions...Spikes, tie plates, fishplates (real rail joiners), ties, ballast, and rail...with additional parts at turnouts. Commercial track does not include fishplates or ballast (on the good track)...and people don't really notice the spikes & tie plates...so the tie width and rail size are the most noticeable components. The rail is oversize on most layouts...and people don't really pay too much attention to the ties (especially when burried in ballast). So the track gauge remains the most important aspect. Hence, many people will find HO track to be a perfectly acceptable choice for a Gn15 layout.
So British railfans...and IHC 4-4-0s...are OO scale trains running on HO standard gauge track for convenience. The downside is that the LPP and buildings look too small next to the trains. Interestingly, with my previous 4'1" gauge example earlier...HO track scales out to b 4'1" gauge track in OO scale.
As far as modeling in a scale without alot of commercial support...like On3 in my case (the structures are either very $$$ or unrealistic 3-rail buildings)...isn't about buying everything under the sun. Like earlier model railroads, much of the pleasure comes from building. It really isn't that difficult to build your own railroad cars...and even easier to build your own structures. The more you invest in something, the more you'll get out of it. Running my On3 trains on a simple oval of hand laid track is 100x more enjoyable for me than running trains on sectional track. The difference gets even greater when it is scratch built freight cars...and I see no difference between my models and the stuff in my books. It is sad to me that so many people haven't discovered some of the finer work in the hobby...they miss out on so much fun!
EDIT: Additional confusion comes from people using slang such as N-gauge. While that is sort of correct...as in N scale standard gauge...it causes confusion. For 3-rail scalers...it is a nice qualifier...as much of the older 3-rail stuff was not to scale...or even pure fantasy. That's fine! Is my modeling of the DSP&P in 1884...with extra traffic...any less of fantasy? The downside to referring to it as O gauge is that it might be taken as talking down to...dismissively. I've seen some fantastic 3-rail layouts.