Very nice work. A few notes that I would suggest:
- The wheels look rather small in the 3D drawing. Fairmont, Woodings, Fairbanks Morse, Northwestern and Kalamazoo pretty much used a 16" "demountable" pressed steel spoked wheel. They also offered 14" and 18" pressed wheels but these were pretty rare. Fairmont also offered 16" and 20" cast steel wheels. These were generally reserved for the "A" gang car series. I know spoked wheels may be a little harder to render in this medium.
- Instead of the two down tubes to hold up the rear part of the roof, maybe a more X looking member? Most motorcars used this flattened X as the rear roof support as shown on my 1923 Bangor & Aroostook M-14
I have a couple on my layout. Think one came from Wheel Works and the other was one part of a section house built up.
What puzzles me is that the ones I remember from the 1940-1950s did not have a roof.
This was very common. as the roofs and fronts were actually options and did not come standard as a package from the builders. When you bought a motorcar from Fairmont (largest manufacturer of them) all you got was the "Dog house" (the part that covers the motor and where people sat), the sub frame and the motor. These "bare" cars were referred to as "picnic tables". Roofs and fronts came in several options such as cloth, steel or aluminum. Roofs came in aluminum and were available in two styles, "peaked" and "pizza pan". Full cabs were NOT an option from Fairmont, and were only available as an option from Woodings on their CBI and CBL models in fiberglass, and from Northwestern Railcar on their hydro-stat fiberglass railcars.
Many railroads built their own cabs, fronts and roofs as well. C&O had a very distinctive squarish cab that we have termed "C&O standard". N&W also had a very distinctive angled front cab that looked like a wedge. Grand Trunk also used a similar cab to N&W. Full cabs were pretty much developed by UP and MILW. UP was required to provide full cabs so that crews would have protection from the elements starting in the 1940's. These were dubbed "Union Cabs" because of the requirement to have them from the Union. MILW also started building full cabs for their cars in the 1970s at their Tomah, WI shops, with the cabs being dubbed "Tomah Cabs" Other railroads also built their own variants and some even used wood, such as the Rutland Railroad.
C&O Standard Cab
MILW Tomah Cab
Former UP MT-14 with Union Cab ( now American LaFrance Red)
This may be just a teeny bit off-topic, BUT...does anyone out there have any information about Union Pacific speeders between 1951 and 1956???? Like, what color, what type, what lettering???...PHOTOS!!... 
I will await any information with 'bated breath...
Almost all of them were painted in Armor yellow. As the "aluminum front" option became available from Fairmont towards the 1960s, everything but the front was yellow, the front remaining a natural aluminum color.
The Canandian roads favored using fiberglass cabs. CN and CP used distinctively different styles and colors. CN used an orange that was referred to "Pumpkin Orange" on fiberglass cabs that were installed on Fairmont motorcars. CP used yellow cabs on Woodings CBI and CBL motorcars.
Here are some Pumpkin Cabs. First two are Fairmont MT-19's ex-CN, the green one is a former CP Woodings CBL, next a former UP MT-14 with "union front" but an open cab, etc.
My former 1985 Woodings CBI CP car, assigned to the Ottawa area.
A cars had cabs that pretty much changed over time. They began with "pizza pan" roofs and then eventually moved to peaked roofs. There are some variants, where "pizza pan" roofs were still be manufactured on cars into the late 1970s, such as the A6 and A8 series Fairmonts. Some A8 cars lasted on METRA long into the early 2000's. Some A8's were even equip with an AAR coupler and could move railcars. A6-A8 models were available with 4 wheel drive.
My 1956 Fairmont A5, ex-Quebec Northshore & Labrador RR car (left) and my friend Harvey (RIP) 1980 Fairmont A4, ex CP (right)
My 1978 Fairmont A6, which was assigned to US Steel in Philadelphia. This sports a pizza pan style roof.
I do, but i don't want to ship it from the East Coast.
You can find most for sale actually on the West Coast. The climate out there, and the sure numbers of them purchased by Western roads has them concentrated out there. It is harder to actually find them on the East Coast. In fact most of the cars you will find on the East Coast are of western heritage. Western roads also ran them longer, some into the 1990's, while eastern roads started to scrap them around the early 1970's. However... motorcar clubs are more popular on the East Coast. I looked at freighting a M-19 back in 2002 from FL and it was going to cost me $800. When I bought my A6 in 2010 I picked it up from Saginaw, MI, and it cost me about $700 to get out there and back, not factoring in the side trip from Harrisburg, PA to Cleveland, OH to transport the CHRS speeder.
The motorcar trailers, lorry cars, are one of the only things in the "speeder" line still manufactured by Fairmont. You can still order them, at about $2500 each, although they are mostly steel frame now. Most were wood frame with idler axles. Steel frames, and axles with "hitch brakes" were options. Hitch brakes would activate via the tow bar, as the motorcar slowed down the weight of the trailer car pushed on the tow bar and applied brakes on the trailer car.
