Sn4 should not be considered an appropriate type as 4 foot prototype gauge is almost nonexistent, generally pre-war in the British empire. Proposed as a layout space saver, track space required is between HO and S scale. AF-2 rail and S scale locomotives and rolling stock convert to Sn4s narrower On3 track by changing locomotive driver wheels and relevant truck assemblies. Narrowing of AF or S scale locomotive chassis is required to compliment the narrower driver wheel and axle assembly width. On3 trucks and knuckle couplers are readily available for modifying tenders and rolling stock. Code-125 a more prototypical wheel for rolling stock and tenders are available in diameters of, 20, 24 and 26s. Layout and structures are always S scale.