The railhead town site was constructed on the Paramount lot adjacent to the neighboring RKO Pictures studio. It became the basis for what would go on to become Paramount's famous western town set as seen in TV's Bonanza (1959) and numerous other TV shows and movies. Prior to 1948, Paramount didn't have a western set on its studio lot. A short line of track was laid down that allowed a working period locomotive to pull into town.

Filmed in mid-1947, but not released theatrically until February 1949.

Robert Preston appears in very similar roles in 2 films, both with very similar plots: Union Pacific (1939) and Whispering Smith (1948) .Both movies center around railroading; Preston plays a likable but misguided "heavy" in both; in Union Pacific, Joel McCRea plays a lawman to Alan Ladd's title character, Whispering Smith; in Union Pacific, Barbara Stanwyck plays the love interest, married to one, romantically attracted to the other male leads - in Whispering Smith, Brenda Marshal plays a very similar character; in his film, Ladd has William Demarest as his sidekick, while in Union Pacific, McCrea has Akim Tamiroff and Lynne Overman; in both films, the likable but misguided Robert Preston characters die at the end; both films take place in roughly the same era; in both films the heavy (Preston) was a good friend of the leading good guy (McCrea or Ladd) from way back when; in both films the lead (McCrea or Ladd) tries to get their old friend, Preston, back on the track to decency by reasoning with him; in Union Pacific, the arch demon is played by Brian Donlevy, while in Whispering Smith, he's played by Donald Crisp; there are train wrecks galore in both films; both the McCrea and Ladd character work for the railroads. There are other parallels, as well, so many that it would seem that Union Pacific served as a rough template for Whispering Smith to a large extent, with Preston playing essentially the same, or very similar, character roles in both films.

"Screen Director's Playhouse" broadcast a 30 minute radio adaptation of the movie on September 16, 1949 with Alan Ladd reprising his film role.

After Whispering Smith gets shot in the harmonica, he comes to in a bedroom (about ten minutes in). He sees a glass of water on the nightstand, takes a sip and fails to put it back and it falls and breaks on the floor. Miriam enters, walks to the head of the bed to greet Smith, where the broken glass shards should be all over the floor, and doesn't step on a bit. And no one picks it up.

Continuity

When Luke gets punched in the face and begins to fall, the chair starts to tip to the side. However, it is standing upright in the next shot.

Just after 23 minutes into the film, Whispering Smith is given a harmonica, the box of which he drops to the ground, only to have it immediately reappear in his hand.

About 7 or 8 minutes into the movie Alan Ladd is reading a telegram that had been crumpled. Then it cuts to a slightly farther shot and everything is still in about the same position but now the telegram has not been crumpled.

When Murray punches Luke, he falls sideways off the chair. However, in the shot of Luke landing on the floor, he lands on his back.

When Luke Smith is pouring coffee for Bill Dansing in front of the Boarding House, one instant he is pouring coffee into Bill's cup and the next he is pouring coffee into his own cup.



Revealing mistakes

Whispering Smith (played by Alan Ladd) is sitting on a high porch. He reaches into his pocket and takes out a harmonica in a box, which he throws away. After playing a tune on the mouth organ, he goes back into his pocket and takes out another box in which he puts the harmonica.



Crew or equipment visible

When Smith shoots his horse, the shadow of the dollying camera can be seen crossing his arm.