I really like those early trials of streamlined motor cars built by McKeen.
I intend to built a SP combo (70' McKeen passenger motor car and its 32' baggage trailer) as well as UP's rebuilt (conversion to gas electric) M23 or 24 with thew matching 51' passenger trailer in 1/29 scale.
I started with the 32' baggage trailer, using an Aristo Sierra coach - the trucks closely match the original McKeen.
For the 51' passenger trailer I'll use a Bachmann coach - its trucks closely match the trucks of those car, too.
I'll use the original models' shells as a substructure on the straight sides and add a plastic or cardboard substructure for the rounded ends. Then I will cover the basic shell with glass-fiber layers and resin.
The McKeen motor cars will be built upon shells of Aristo heavy weight cars with additional subconstruction for the knife-sharp or parabolic front and the rounded end.
UP's rebuilt McKeen will get an USA Trains NW2 power block for the front truck, the SP will get the same truck on the rear, making the front truck with its different wheel diameters probably a dummy (or only a single driven axle).
Pictures will follow when I have some basic progress to show...