Hmmmmmm…
An LGB car, passenger trucks, a smoke stack and paint/Decals.
Who would know the difference? 
Good idea!
Might be good to cut 3 of them into 2 longer versions?
I just happen to have 5 of those green cars.
I’ll put it on the list of possibilities…
The LGB cars are a good choice, being a modeler I would use then and make the upgrades as needed. I don’t know how hard these cars would be to find today at a reasonable price, but I would watch eBay as they do come up from time to time.
Isn’t there a cheap Newqida version? No longer available new, I guess.
I’m pretty much with your thoughts on that John.
I’m not sure how long the LGB cars are but using round numbers and say in 1/24 scale if the cars are 25" long then you are pretty much spot on. Not sure if it’s worth the effort or cost to even extend them.
I guess it depends on how accurate you want to be. Myself, I’m not so much.
I have some LGB western style ( D&RGW, WP&Y DSP&P) that aren’t accurate.
But I like them anyway.
So if I did this, I wouldn’t chop and extend any cars. (The actual round roof WP cars are long.)
This shot shows one in between the regular cars,
https://wpyr.com/wp-content/uploads/2014/03/rolling-stock-coaches.html
This is why I went with the round numbers I did. 50’ is a good starting place and you decide from there?
As you know I model by eye myself and I was agreeing with your initial thoughts.
They sure rostered a lot of passenger cars, didn’t they!
You could model Sumpter Valley RR as well, because they ended up with a couple of the WP&Y round roof cars.
There’s a few Newqida’s on eBay. (I had to look, Pete)
Cheap enough, but the roof flattens out on the ends.
I’m intrigued with the WP&Y Beehive Carriages.
The train wasn’t running when we were there. How does the trip compare with the D&S?