Is OO gauge the same as HO? Yes and no. How is that possible? Here’s how and why they can be the same and different.

To clarify: HO and OO are not the same scale. But they are sometimes, dare I even say usually, the same track gauge. Let’s see if I can clear up the confusion, because there’s an exception that’s a bit obscure but can cause problems on either side of the Atlantic ocean.

OO gauge versus HO

Is OO gauge the same as HO

OO gauge is usually the same as HO, even though the scale is different. But this American oddball, late 1930s Lionel OO track, is 19mm wide, versus 16.5mm for HO.

HO scale is the most popular model railroad scale in the United States, continental Europe, and most of the rest of the world. OO scale is the most popular model railroad scale in the United Kingdom. HO scale is 1/87 scale, at least when we’re talking trains, which means it is 1/87 the size of the real thing.

OO scale is 1/76 scale, which means it is 1/76 the size of the real thing. Take a model of the same 40 ft box car, and it will be about 5 and 1/2 inches long in HO scale. A OO scale model of the same box car would be 6.3 inches. OO scale is about 14% larger than HO scale.

What about gauge, though?

So far I’ve been saying scale. Gauge isn’t the same as scale, and especially in this case. Even though the British decided to use a slightly larger train scale, they adopted the same track the rest of the world uses for HO scale. So as long as you are talking about British OO, it is indeed the same track gauge as HO scale.

Sometimes you’ll see figures or structure models sold as HO/OO scale. When it comes to figures and structures, there’s enough variance in the size of the real thing that the 14% difference in a model isn’t necessarily all that noticeable. The actual scale may be one or the other or something slightly in between.

But of course there is an exception. Would it surprise you if I told you the exception is American? And one of the usual suspects at that?

Lionel OO scale and gauge

In the late 1930s, Lionel Corporation decided it needed to modernize. So they scrapped their nonstandard Standard Gauge trains, introduced T rail O gauge track, and they wanted a small, modern scale train product line. Although HO scale was already gaining popularity, the slightly larger OO scale had a niche following in the New York area, where the Lionel CEO lived. This may have caused him to overestimate the popularity of OO scale. Whatever the reason, Lionel introduced a small line of OO scale trains in 1938. Along with it, they introduced track for it to run on. It had Bakelite roadbed and either two or three rails, because they made the trains both ways, and the track gauge was 3/4 of an inch, or 19 mm.

These trains were on the market for about 4 years and sold poorly by Lionel standards. Lionel stopped making trains in 1942 because of World War II, and Lionel didn’t resume production of its OO line after the war. Eventually they decided to try their hand at HO scale.

The Lionel OO track can be a bit difficult to find today, so it’s tempting for a Lionel OO hobbyist to import British track. But it won’t fit. I know someone who tried. And if that prewar American take on OO gauge track seems interesting to a Brit, they’ll find it’s too wide for their trains.