SL500 SL500 O-16.5 Narrow Gauge Code 100 Nickel silver flat bottom rail (Wooden type sleepers)16. 5mm gauge track in 7mm scale gives an equivalent prototype of 2ft 4 ins, making it a popular choice for modellers, this scale is also known as 0n30 in the USA because this results in a prototype gauge close to 30ins. many modellers find that this scale gauge allows an ideal combination of scale detail in a relatively small space. Technical Specification: Length: 914mm Box of 12 Lengths Due to the Length of the track we cannot use Royal mail for