The idea for the association and the two key players were Burlington and Santa Fe. Missouri Pacific joined because they saw the other two railroad properties starting something and they didn't want to be left out. Safeway of Illinois and Martz were recruited by Burlington and Santa Fe to provide a feed from the east for their buses at Chicago.
The reason the Trailways "idea" caught on so fast is in one word, Greyhound. In the late 20's and early 30's, Motor Transit Management (Greyhound) was busy putting together a nationwide system of bus companies using the Greyhound name and the public was beginning to recognize Greyhound as a company who could take you coast to coast. While Greyhound wouldn't finish buying out the railroad interests until the late 40's and early 50's, they succeeded in getting the name out there.
Trailways was an attempt to form a non-profit operating trade association to spread a common name coast to coast with each member representing the other members as if it were his service. Worked too. Most people fail to see that the secret to making Trailways work is to lose yourself inside the association. Best example of that were the east coast carriers during the 60's-70's and 80's when all you saw on the bus was Trailways except for the certificate lettering on the baggage doors. During that period. with the exception of Continental, all the member company's buses were just marked "Trailways," Martz being the sole exception.