scale mile in HO - Model Railroader Magazine

I wouldn't rely on scale miles on where to put your signals at. Our club layout has about 4 scale miles of track on the main and we have the second largest club layout in Oregon. If you were to place a signal one per mile, you'd only have 4. In contrast, we have about 10 signals, most of which are in separate scenes and may be anywhere from 10 to 30 (actual) feet apart. I railfan in Albany, Oregon, and there are 3 signals all within a half a mile strecth. Given also, they are signals within yard limits.

In short, although it seems realistic to place one signal per mile, often times, they are closer together.
hope this helps
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