Feeling the effects of the Depression and declining business, America's railroads (in the 1930s) were looking for ways to reinvigorate passenger travel. As Ralph Budd, president of the Chicago Burlington & Quincy, later explained, railroads had to continue running trains on short routes to handle mail and baggage "whether or not anyone rides the trains." After seeing GM's powerful diesel engines, Budd concluded that what the railroads needed was a new kind of train that was fast, convenient, ultramodern and luxurious enough to fire the public imagination. The Union Pacific Railroad also saw the two exhibits and came to similar conclusions. A race was on to see which of the two railroads would be the first to develop an ultramodern railcar
Union Pacific М-10000 City of Salina weighed 20 per cent as much as a conventional railroad car, but using only two minuscule (by railroad standards) 320-hp six-cylinder truck engines, was able to hit 100 miles per hour, while delivering 5 miles per gallon. By comparison to conventional railcars, the ride was superb, engine noise and fumes were all but eliminated and the seating arrangement - using aircraft-type seats as fitted to the Scarab automobile - made the Railplane quite luxurious
In 1933, the Pullman Car & Manufacturing Company constructed the Railplane to Stout's design (some improvements were later patented by the company, see the positives below). This was merely Stout's familiar triangulated space-frame aircraft fuselage, this time adapted to railroad use. Here too, he was able to preserve his all-time important triumvirate: simplicity, practicality and comfort. The self-propelled car had an aluminum body, 60' in length. It was exhibited at the Chicago World's fair 1934 and then leased to the Gulf, Mobile & Northern in 1935 for service between Tylertown and Jackson, Mississippi. From the railroad point of view, all running gear could be easily serviced from outside, tracks and roadbed lasted longer and operating costs were significantly less. Despite proven advantages, there were no buyers. Union Pacific ordered a three-car version (future City of Salina) , but that's as far as it went.
from http://www.dieselpunks.org/profiles/blogs/sunday-streamline-12-pullman and http://www.dieselpunks.org/profiles/blogs/flying-americans