I posted this picture in another thread, and thought it might be worth a comment or two. I don't want praise (well, everybody likes it) and I'm not trying to show off. I hope that I can show somebody out there that it is really not that difficult to build a steam loco from scratch.
Here's the photo:
I'll describe the basics, trying to be brief, and see if anybody wants to know more. PLEASE feel free to ask!
Tender
The tender is folded up from a single sheet of .010" brass for the sides, with another forming the top of the tank and rear of the coal bunker. Inside are square brass tubes for rigidity, both top and bottom. I left the front and top open, for adding sound later (my speaker fires forward into the cab). The frame is just square brass tubes with brass sheet soldered to it. The I beam is made by recessing the edge of the tube slightly. The bead is some 28 gauge brass wire from the local hardware store, liberally soldered in place in one continuous line.
Rivets
The rivets are punched, one at a time, with a sharpened nail in a pin vise, working against a smooth hard surface (glass or polished metal work beautifully). I put a sheet of typing paper between the brass and the surface. When I press the point into the brass, it will sink just the thickness of the paper, and not go through because of the hard surface underneath. When making long lines of rivets, I work against a metal straightedge and just eyeball the really close spacing.
Boiler
I form my boilers from sheet styrene. First I find a piece of PVC pipe (or metal, if I cannot get plastic) that is as close to the perfect diameter as possible, minus at least .060". I cut two rectangles of .015" styrene that will form a tight tube around the pipe in two layers with no overlap at the seams. Using an ultra fine Sharpie, I mark the larger of the two into longitudinal quarters, and across for things like the smokebox length, stack location, etc. I then wrap them very tightly around the pipe, with seams 180 degrees off, and tape the seam LIBERALLY with 3M Scotch tape (the clear kind) The tape will stretch and pull tightly enough to close the seam completely - I usually just start wrapping and keep it just as tight as I think I can without breaking the tape. I then put the whole mess in a cup of water and cook it in the microwave for about 5 minutes. When the water cools (or take it out and run cold water over it) the sheets will have taken the form of the pipe, and the datum lines will still be there (note the blue lines on the boiler in the photo). I then build up layers to form tapers, larger diameters, fireboxes, etc., always drawing the datum lines when the sheet is flat, or transferring them as I add new material.
Stack and Domes
These I make from tubes and sheets. I start by cutting a circle in .010 or .015 styrene (oval, to be 100% accurate) the size of the base of the flare on the boiler, and glue it EXACTLY where the stack or dome will be. I lay masking tape over the circle, and cut around it, leaving the tape with a hole in the middle. Then I build up the stack or dome from tubes, and glue it into place. I spread putty liberally around the joint, and whn dry I sand and file it to a smooth curve. The tape protects the boiler, and the edge of the styrene circle shows though the putty to show how even the flare is. I make the tops of the domes by gluing up stacks of sheet to whatever thickness needed, then carefully hand sanding and filing to a smooth curve. The stack is tapered on a bamboo skewer, using sandpaper and a file, then a solid circle is glued to the top. When the glue dries, I drill out the center of the stack, and file the edge as thin as I think I can get it.
Cylinders and Saddle
I make my cylinders first, by cutting tubes to length and gluing on caps. I cut the caps using a compass, which leave a nice dimple in the center, perfect for modeling the cap nut. When I've cleaned the joint well enough that the cylinders look like solid pieces, I glue them to a cross piece (which is usually T shaped, the front being the pilot deck and beam attachment). The valve chests are simple styrene boxes. the saddle is a simple box as well, except having a top which is curved to fit the smokebox diameter. Piston valves cold be made up in a similar fashion, and then wrapped with either .010" styrene or brass.
Cab
The cab is made just like the tender. Cutting the window openings is a little tricky, because you can't use shears. I find that a very sharp hobby blade (actually two - a blade with the point broken off and re-sharpened, and a chisel blade) work well for slicing and chopping through the thin brass.
That's all there is to it!
I think that pretty much covers the basics in a nutshell. As you can see, I made my own side rods, main rods, crossheads, guides, crank pins, etc, but you can use a commercial drive there if you prefer. If you can figure out how to make the rest of the model, you can probably figure those out as well. Remember that a locomotive is just a bunch of pretty basic shapes stacked together. Learn to see the shapes, build one at a time, and glue them together.
Oh, yeah.. That engine took me right at a year from start to finish, working in my spare time.