a five inch gauge ajax steam locomotive ajax a five inch gauge steam locomotive designed by dick simmonds running at the romney marsh model engineering society The Ajax is a popular model steam locomotive, and in the context of model railways, "five inch gauge" refers to the track gauge or the distance between the rails. In this case, it means that the model locomotive is built to run on a track with rails spaced five inches apart. Model locomotives are often built to different gauges, such as 3.5-inch gauge, 5-inch gauge, 7.25-inch gauge, and so on.