Western Mail

1942. From iMDB:
Just as Lucky Webster (Fred Kohler Jr). is being questioned about a recent train robbery, Tom Allen (Tom Keene) appears at his ranch and confirms his alibi. Lucky offers Tom a job with the gang he is secretly working for. Lucky's sister, Julie (Jean Trent), doesn't realize he is mixed up with an outlaw gang. Tom, a U.S. Marshal working undercover with Sheriff Big Bill Collins (Glenn Strange), asks the sheriff about respected citizen and bank president Jeff Gordon (LeRoy Mason), whom he suspects of working with the mail robbers. Tom plants a dictophone in Gordon's office and hears him tell gang leader Jim Rivers (Karl Hackett) that the next shipment is $15,000 and gives Rivers information to make the hold-up. Julie, accidently, tips Lucky off to Tom's real identity and Tom is ambushed as he rides back to the gang's hideout.