When you say you are modeling in HO scale what does scale mean and what is the difference between scales and gauge? In model trains began with what was called Carpet Railways.
These were built usually out of brass, and were basically a boiler on wheels.
The boiler was filled with water and the burner was lit.
The engine ran until it ran out of fuel or hit a piece of furniture sometimes turning over and spilling flaming fuel over the floor.
These trains had no scale they were whatever size the modeler made them.
Electric trains appeared around the turn of the 20th century, but these trains still had no certain size or scale.
Even after manufacturers began making commercially produced model there was a difference between model sizes according to the maker.
It could be very frustrating to buy the track and maybe the rolling stock from one company saying it was O scale, and buying the locomotive from another manufacture as an O scale.
You get the locomotive home and it would not fit your track.
The NMRA (National Model Railroad Association) was formed in 1935 in the USA.
They started trying to get a standard set of rules set up to govern the manufacturing of model trains.
Also the MOROP was founded in Genoa Italy in 1954 that began setting standards for European manufacturers.
The scale of a model is usually defined by the ratio of the model to the real prototype.
For example, Z scale in the USA has a ratio of 1:220, with N scale being 1:160, and HO scale 1:87 ratio.
Even with these rules in affect there are still some differences in scale.
In O scale the ratio can range from 1:43.
5 for models in Great Britain and France to 1:45 in Germany and in the USA the ratio is 1:48.
Even though you have these different scales in the O, they all run on the same gauge of track 1.
25 inches between rails.
The chart below gives you an idea of gauge and scale.

Gauge Designation Space Between Rails Scale (fractional equivalent) Standard ** 2.
125" (54mm)1:24** G1.
75" (45mm)1:22.
5 or 1:20.
3 No.
11.
75" (45mm) 1:29 or 1:32 O**1.
25" (32mm)*1:48** Proto-481.
169" (43mm)1:48 S .
875" (22.
5mm)1:64 OO3/4" (19mm)1:76 HO .
650" (16.
5mm)1:87 TT .
471" (12mm)1:120 N .
354" (9mm)1:160 Z .
256" (6mm)1:220 * O gauge measures from the center of the two outside rails.
** These gauges of track are used by toy trains without much attention to scale.
Closest approximations would be as shown, though they can vary quite a bit.
So you see that there is a big difference between scale and gauge.
When you begin to model your train layout always make sure that the ratio is the same if you want everything in proportion.
Especially when it comes to automobiles and other equipment you need to be careful there is a great range of sizes that claim to be the same scale.
Just remember the main thing about model trains scale is to always have fun modeling.
Thank you, Steve Barnett