At the end, the train that the suspects were boarding to return to Chicago was a Southern Pacific (SP), which runs mostly within California and along the border with Mexico into the Southern States. In the 1950s the most direct and quickest route to Chicago would be on a Santa Fe train, which also had many more stations then the SP did.
Several things would exonerate Sgt. Williams. Notably, his phone call to the station from his home compared to first reports of the crash could be used to prove he could not have been the driver based upon time and distance. Additionally, the imposter was not wearing sergeant's chevrons on his uniform. Next, when Mathews interviewed the witness, Williams was present and the witness looked at him and spoke to him. She never identified him as the driver. Finally, since she removed her gloves, the fingerprints of the woman would be present in Williams' apartment to support his version of the events.
When Sgt. WIlliams tells Dan Mathews about a friend of his Aunt Audrey, Mathews correctly refers to her in conversation as a friend. In the final line of dialogue (see Quotes section), Mathews refers to "Aunt Audrey's relatives".