Those dimensions and weights you give are in fact the dimensions of the car I am building. The original car is at the NSRM Boulder city, and I have taken all measurements off of it, including the types of wood used. There are only two differences: First, the seating will be in chairs and not the throw overs. Second, the original car was built as a full coach. I decided on a combine, patterened after its current configuration, for practical reasons. We are always looking for a place to put things on the road, and a combine is the answer as it was for the E&P. We may not be hauling express for Wells Fargo, but we still have stuff to carry. These include camping equipment, sleeping bags, tools, wool waste for journal boxes,packing, food, dogs, jacks, you name it.
As for the speed of Eureka, there are suitable railroads on which I have tested it. The particular railroad we did the test on was the U.S. Gypsum railroad. Back in the early 90's I had her up to 40 mph which Baldwin advertised as top speed. That railroad has 100# rail or bigger. It was like being on the interstate. The locomotive is very well balanced, and could go faster. However the limiting factor is the tender. The water in the tender is not baffled, and it eventually sets up a side sway that literally pitches wood over the side if high enough.
When the E&P was new, it was probably a pretty good road to run. But, even then I would not want to go 40 mph on 35# rail. It had to be a wild ride and those guys had brass cajones. I guess they were the railroad version of barnstormers.
Dan