Over the course of many, many years of designing model railroads, and especially O Scale Three Rail, I cannot even count the number of times I get a request for O72 Curves.

Often times the request will be on the order of ‘I must have 072 radius curves’. This, of course, is a misnomer in itself: O72 refers to the DIAMETER of the circle, the curve radius is actually 36”. Many times I was called to task by my customers with statements like “36” radius is much too small, I must have at least O72 radius.” Also, sometimes related to comments like I have to run my O scale Big Boy on my 4’x8’table, but that is another story.

I am fairly sure as to the beginnings of the idea that one must have O72 for larger equipment. For its large O scale trains, Lionel developed a machine that pumped out O72 track. From that time on, O72 became a sort of ‘standard’ not only for Lionel, but for anyone manufacturing a large O scale locomotive. “I have large locomotives; therefore I must have O72 curves.”

With the use of flexible track, especially Gargraves, it is not necessary to hold to a 72” diameter curve. And, with a little tweaking, even down to a 060 Curve (30” radius will work quite well for most O gauge railroad equipment. Even taking just a few inches off the radius (or diameter, if you prefer) can offer large space rewards in your layout design.

Now, that being said, when incorporating a dual track mainline into a layout, we use a 4-1/2” separation between tracks on the straight. On the curves we then expand that separation on curves for an O72 – O60 combination. With the O60 curve, many design possibilities are opened up, especially when you have a figure eight type of scenario where each line must transit the O60 line to complete a figure eight. (each line goes inside, then outside of the figure eight)

Want to run a Big Boy, or any of the other HUGE locomotives? Well, then yes, and you might even want to consider O96, or O120. But for most of us that live in the real world, park the Big Boy on the mantle, or perhaps a stall in your roundhouse, and run everything else on your layout. By the way, you CAN run a Big Boy on a 30” radius curve(O60). Clearance is the issue, both in appearance and physically knocking down things along the right of way.

Another thing to remember is the prototype. There were only 25 Big Boys. Yes, that is correct, 25. And they were run on only one railroad, the Union Pacific.

I mentioned earlier that we like to use a 4-1/2” separation on the straight for parallel tracks and a 6 inch separation on the curve. One of the tricks you can use on curves is to expand the distance between parallel tracks, as shown in the picture. If you use a normal separation of 4-1/2 inches between parallel tracks (center to center), you could use a 30” radius on the inside track, and then expand the outside curve to 36: (O72). This will allow for large equipment to run side by side.

Shrinking the radius requirements on your layout can give you a lot more operation and variety to your layout. You will find it amazing what can be done with a few inches saved here and there.

Don Cardiff
9/02/2009