Peco Streamline SL-500 On30 Code 100 Narrow Gauge 36" Flex Track with Wooden Ties 12 Pack 16.5mm gauge track in 7mm scale gives an equivalent prototype of 2ft 4 ins, making it a popular choice for modelers, this scale is also known as 0n30 in the USA because this results in a prototype gauge close to 30ins. Many modelers find that this scale/gauge allows an ideal combination of scale detail in a relatively small space.