This is not a flip answer but it may depend what kind of O gauge three rail layout they have. If 0-27, vehicles from 1-34 to 1:53 or even 1:64 may look just fine. Remember that some pieces of 0-27 rolling stock have been used on S gauge(1:64) layouts by changing the trucks. If 3 Rail Scale(3RS) the modeler may want only 1:48 vehicles. If a vehicle is not available in1:48 they might use 1:50 models which are only four percent smaller or 1:43 models which are ten percent larger. Use actual dimensions rather than the manufacturer's description when considering vehicles. Some modelers carry a card in their wallet with marks at 1 1/2, 2 and 2 1/2 inches indicating 6' for an average man, 8' for a 96" wide semi, and 10' for the width of a rail car. If this is for your layout, use what you want. John in Lansing, ILL