Hmm, probably not impossible, though honestly not a project I'd likely do anytime soon. You could probably balance the frame on the front and rear truck with springs, much like the prototype. With models though you could probably get away with having it rest on a stiff drawbar to the tender a bit.

Before the cab was started, putty was added to make the base of the sand dome and stack. This is done before the cab so there is room to sand.
Before and after sanding...



The pencil mark on the boiler in the after sanded picture is to mark where the front wall is to sit for the cab. This is made easier by using a mantua camelback cab from the parts box as a bit of a template. Parts like these in general are very useful for reverse engineering.


Getting ahead of myself a little attention went next to the running boards (which the cab sits on) but, to place the fireman's side properly the air compressor was placed since it sits so low on the running boards. The boiler mounting bracket that comes with the calscale single air compressor was used up side down to have the bracket rest on the boiler properly making a triangle mount.



The .040 for the running boards are cut to not exceed the width of the firebox. First glued to the front of the firebox and the fireman's side had the added glue point being able to tack it to the compressor itself. The engineer's side just needs to be looked after until set again glued to the front of the firebox. Once the cab is on the running boards are pretty solid.


Using tube stock the steam delivery pipes are fitted which is a lot of test fitting and trimming until it looks right using a slow setting glue to tweak as needed. Then .040 running board pieces are notched to fit the pipe.



The front and rear walls of the cab are made by placing the boiler face down on .040 and tracing the outer dimension of the front, then find the center of that circle. having the running boards set you can easily measure the width which should be around the same as the firebox width, but .020 skinner to account for the cab sides. Using the center line on the circle made from the boiler front center the width measurement (half the total either direction) and you have a box centered on boiler. Using the mantua cab for measurements you can set the heights. After the basic shape is cut out the door openings are cut. The walls are then glued to the boiler, I use testors glue (red tube) for this because it does not set quickly and will help fill any minor gaps.


Trying something a little different I did the quick gist of cab controls and painted the cab interior before putting the cab sides on to help streamline the process and it makes it easier to paint.


The .010 cab sides are made again using a mantua cab as a bit of a measurement template and the window frames (that the crew didn't take out) are placed, also .010.


After the sides are placed I have been adding some .040 supports to strengthen the roof and the top of the cab side from warping over time which can happen. Also the train control box was added for no reason, an optional part.




The roof is another part with a lot of test fitting before gluing down, the underside is also painted before gluing. The roof detail is then added, made mostly from .010 except the .040 spacer of the center hatch.


The rear running boards were then added cut to fit in line with the bottom of the windows of the cab and not to exceed the width of the firebox.


The rear shade or second cab starts like the center cab tracing the firebox to get the arc line. The width was eyeballed from the tender about as wide as the coal chute opening. Here is the test fit.


Another piece with a ton of test fitting the roof is bent and fit around the base glued to the edge of the firebox. The marks on the piece are what will be trimmed off.


The piece is then trimmed and glued in place. To hold the shape in the rear a little .040 support is fitted towards the rear.




The air tanks were then fitted using tube stock. I tried a new method of capping the ends and sanding them round like the top of a dome. .010 was added to act as mounting straps as well as small .040 triangles to act as supports.
This is where the project sits now.

Since the major construction is done it is time for a lot of little things. I have been experimenting on different ways of making my own washer plugs, these are the most recent incarnation. Started by drilling a hole in rod stock then backed with .010. A little piece (a cube) of .040 is added in the center to act as the plug head. Then the piece is glued pretty flush with the boiler, I liked setting them a little out then trimming or sanding down to the boiler level.

Also boiler bands were finished and the trim on the rear shade was added.


While things were drying on the plugs the rest of the cast parts were added on to the boiler, bell, saftey valves, whistle, check valves, marker lights and front air hoses. The only major detail on the fireman's side is the compressor.


There is a little yoke/crosshead guide support on the engine, this was made from .010 glued to the link hanger frame. The piece is not connected to the crosshead guides so there is room for the crosshead to move all the way to the rear.


The Reading style smoke box step was added, .010 and while I was at it the steps behind the cab were too.



The steps off the rear running board are placed about half way down the back head made of .010.


The final piece of the rear of the boiler added were the two fire doors, I just make some pieces to represent them, since they are somewhat hard to get a hold of and are a part hard to see, especially since I put the cloth curtains on.


The last thing added were the pilot steps, following the prototype they stick back rather far so the bottom step is trimmed a little on the inside for lead truck clearance, wanting the engine to run on 22" minimum.


The little things on the tender started with the trim around the top perimeter, using thinly cut .010 strips. Also in the images is a side view of the rear headlight that sits on a piece of .040 supported by 2 small pieces of .020 wire.


Along with the rear head light the water hatch was made, a .040 base capped with .010, the hatch split and hinges are .010 strips.


The front of the coal bunker is made of .010 as well as the split and latches.


The last little bit of castings are the rear hoses and steam heat line.


The rear ladder starts with 2 pieces of .010 cut in extended C shapes to fill most of the tender height. Then those pieces are placed. The spacing was honestly eyeballed if a measurement is needed I would use the spacing of a ladder on another engine I had.



Little strips of .010 are cut then glued into place using something that doesn't set right away to have time to tweak the rungs into place. Then a little rear number board was added.



The engine is now ready for some decal rivets, then final detailing. I will likely be doing some test running too just to make sure then is no real unforeseen problems before the more delicate plumbing is on.

Is that copper or bronze wire you're using for the cow catcher and other piping?

It is bronze wire, .020, bronze over brass because there is a stand at the Timonium train show that has it a little cheaper then brass. I have noticed no difference in performance.

By the way, Aristo-Craft made a model of a P&R 4-2-2. The real engine was unsuccessful and sold to the Eames Vacuum Brake company to pull a demonstration train. I found one in bad shape at a train show. The boiler will eventually become a early B class 0-6-0.