Scale is the proportion of the model to the full size prototype.  Gauge is the distance between the rails.  

In 1891, Marklin originated Gauge 1.  It was the first gauge for which a series of matching trains and track was made.  Originally 48 mm from center to center of the rails, it later became 1-¾ inches (45 mm) between rails in the United Kingdom.  

Aster live steam locomotives are designed to run on 45 mm gauge track.  Years ago, Henry Greenly in the United Kingdom standardized 45mm for Gauge 1.  Recently it has become a popular choice for garden railways and with small-scale live steam enthusiasts.

In Gauge 1, models of Japan National Railway (JNR) locomotives are 1/30 scale.  Japanese prototypes operate on a standard gauge track that has a 1067 mm gauge.  European and American locomotives are designed for a standard gauge of 1435 mm (4 ft 8 ½ inches). Aster models of European and American  locomotives are 1/32 scale.

In 1989, Aster changed the back-to-back dimensions between wheel flanges from 41 to 40 mm so that locomotives could negotiate smaller radius curves.  This fact should be taken into account when designing track work.  The following Aster locomotives were built with a back-to-back dimension of 41 mm: Schools Class, King Arthur Class, Americanized Mogul, Old Faithful, Reno, PLM 231, Narrow Gauge Shay, JJNR C12, SNCF 141 R and JNR C57.  On all other Aster locomotives, the back-to-back wheel flange dimension is 40 mm.

Live steam locomotives are built in other scales and gauges.  However, Gauge 0 track and Gauge 1 track remains the most popular for small-scale railways.  Practical limits that affect small-scale live steam locomotive are minimized in Gauge 1.  A few Aster locomotives are capable of hauling an adult, but this is generally not practical because of track strength limitations.