The Laughing Policeman (1973, Bruce Dern, Walter Matthau)

"Dern and Matthau search for a mass murderer who shot up a San Francisco MUNI Mack. This event occurs near the beginning of the film. At the end Dern is driving a car trying to find the Mack bus that Matthau is on...along with the killer. Several MUNI Macks appear before Dern saves the day just in time." (thanks Bruce Korusek)