McNally plays Steve Davis, who's working undercover for the U.S. government, trying to infiltrate and bring down a gang robbing brand-new mail trains. Congress is threatening to cut off funding for the trains if the rampant theft cannot be stopped.
Smith is Mary Williams, a glamorous saloon singer; she and Steve strike sparks every time their paths cross.
Mary thinks Steve's a bank robber on the lam, but she's so attracted to him she doesn't really seem to mind, other than worrying about his safety. Besides, she's not exactly angelic herself.
There's a too-long passage where Steve's undercover in a horrible prison run by Warden Haynes (Ed Begley Sr.), but otherwise this 87-minute film runs along at a pretty good clip, with a couple unexpected twists and turns. The movie's nothing spectacular, but Western fans will likely find it pleasant viewing, as I did.
Much of the film was shot on location in Technicolor by Russell Metty. The bad guys' hideout among the cliffs looks like it might have been filmed at Red Rock Canyon, seen most recently in DAKOTA INCIDENT (1956). The hideout, which must be accessed via a series of ladders, gives the film a distinctive look.
A commenter at IMDb mentions having been present when the company was filming in Sonora, California; I suspect that's where some of the train sequences were filmed.
The supporting cast includes Howard da Silva, Dan Riss, Roy Roberts, Armando Silvestre, James Arness, Richard Jaeckel, Whit Bissell, Frankie Darro (WILD BOYS OF THE ROAD), and a young Richard Egan, seen by me last weekend in POLLYANNA (1960).
WYOMING MAIL was directed by Reginald LeBorg.
WYOMING MAIL has been shown on TV in the past on the Encore Westerns Channel. It's not available on DVD or VHS in the U.S., but it's had a Region 2 DVD release.
At the time of this writing, the movie can be found on YouTube.