A surface gauge is cheaper, and you can set it against a rule.

Neil

Use a 3 foot long rule, a trammel and a square. A 3 foot high height gauge would cost a fortune and would be a pain.

I would suggest start with a decent square,a foot rule and a 3ft rule ,a new sharp scriber,some layout fluid, thats all you need to mark out a set of frames and hundreds of locos have been made with these simple tools, you then also need to be able to file true to a line,and file an edge straight.Its no good having loads of expensive kit and cannot file,of course you may have a milling machine which does make things easier though you do need the ability to use hand tools. A surface gauge or a height gauge can be useful to measure and mark distances from the bottom of the frames up,but of little use along the length of the frame,with these tools an angle plate and a flat surface are required.On a loco frame the most important thing to achieve is that the axles are square to the frame and are parallel to each other,to ensure the wheels are free running. do remember a locomotive is probably the only open crankshaft engine where the crankshaft is sprung and the wheels and crankshaft follow the track,so parallism reduces friction but precision fits will cause to much friction or bind the engine solid.