Semi vs Scale - How to tell?

For Freight Cars - MTH Purple Box (Premiere), Atlas O, Weaver, CMP, Intermountain, Old Atlas Metal Kits.  That's what I have bought, because those all are definitely Scale.  Beyond that buyer beware.  With Lionel it is really tough, hopefully the Lionel Experts on the forum can help you determine.  They run the gamut.

For Locos - K-Line their boxes will tell you if they are scale.  Williams the same.  MTH Purple Box (Premiere), Weaver and Atlas O are Scale.  Lionel consult an expert!

For Passenger Cars - K-Line if full length (not plastic Streamliner series).  Atlas O the same.  Lionel consult an expert!

Of course High End - 3rd Rail/Sunset are all scale with the only exceptions being some rare shorty passenger cars.

For Older Items - All Nation, etc. almost all are Scale.  That can get tricky though, consult an expert!

Hope that helps!

For Lionel, there is sort of a way to tell, but even then, you need to be careful. Scale cars will usually have a 7 as the second digit in the number (i.e. 6-17334). Semi-scale/Traditional will have the second digit as a 5 or 6 (i.e. 6-15384 or 6-16284) However, if the second digit is a 9 it could be either or. After some time, you will develop an eye for it and can also look at size. For example, the 6464 boxcar series measures at 10 1/2 inches and tends to be the most popular you will see. I'm not great with passenger cars but 18" and up are scale. I look the same way you do with comparing trucks to the overall size of the body. I've made mistakes too

Only real way is to compare to prototype dimensions.  What's on a box or a label means nothing.

Also nonsense is this thing about passenger cars being scale if they are 18" and up;  passenger cars came in all sorts lengths down to 34'.

The best way to compare the model scale dimensions to a prototype drawing as MWB mentioned.    Actually, an equipment diagram generally has enough dimensions on it, so you don't really need a drawing.   Generally you need to compare the length at the end beams because most couplers on models are not scale or are not mounted in scale length from the body.    You also need to compare the distance between truck centers, the overall height, and the overall width.    That really gets you in the ball park but dimensions also help.    After that you can compare the model to photos too.

But to compare the measurements,  you need to have the  model in hand to measure it. 

Any of the cars in the various WEaver lines are scale.    They were all based on real prototypes and as far as I know are scale.

For the Older stuff, Athearn kits (which became Reynolds, and maybe Old Pullman), All Nation KIts, Quality Craft, Gloorcraft, and Walthers kits are scale and based on prototypes.    Theses kits when built up well are quite nice models.     I don't know the older Scalecraft and Westbrook lines.  

Passenger cars are more difficult because there is so much unique variety.    And even Walthers played some games with their "Shorty" kits which were also available as a lower profile to match Lionel Madison cars.   But the same reference to prototypes does work.

OP @VJandP. The answer to your question really depends on how you are/want to model. If you model prototype the post @prrjim made is completely correct. You find the actual train/rolling stock in real life and get the actual dimensions and then compare those to the train/rolling stock model to find out if it is scale at 1:48 or not. This can lead to all different sizes being considered scale because of all the shapes/sizes of cars that exist in real life.

However, I'm assuming your question is something that I ran into as well during my time in the hobby. My experience was different, but I didn't know that there were standard 'O' and traditional 'O' rolling stock sizes. If you are looking to differentiate between standard 'O' vs traditional 'O' for Lionel, what I posted gives you a guide to start. As you noticed, MTH makes this easier. I'll paraphrase from the MTH site:

"RailKing are smaller models. RailKing items are identified with item numbers that begin with "30".  RailKing Rugged Rails items begin with "33"."

"Full O scale models, 1/48 the size of their prototypes M.T.H.'s Premier items are identified with item numbers that begin with "20" for Hi-Rail models with item numbers ending in "-1" and "22" for Scale Wheel models."

Hope that helps.

Thank you everyone for the great info.   It will help when plucking through dozens of for sale items, especially tips the Lionel. 👍🏼

@VJandP posted:

Thank you everyone for the great info.   It will help when plucking through dozens of for sale items, especially tips the Lionel. 👍🏼

Anytime. Incredible layout btw. Really, really like the track plan you used.

@MattD87 posted:

Anytime. Incredible layout btw. Really, really like the track plan you used.

Thanks @MattD87 .   I’ve learned a lot with these 3 simple loops thanks to this forum.   The goal was to learn as much as possible before building a larger layout in the new house next year,