(1950, Marshall Thompson)

“Thompson rides a Greyhound Silversides bus. At a rest stop he apparently takes the driver's gun which he spots in the front of the bus. At the end of the trip the driver asks him about the gun and is shot.” (thanks Bruce Korusek)
The Silversides in question is Greyhound 816, a 1948 GMC PD3751:

As it pulls in at the 'Terminal City' bus station, it's followed onto the stand by an older Greyhound, 663, a 1937 Yellow Coach 743 'Supercoach':