L-to-R are unidentified, Jack Rockwell, Tom London, and Charles 'Slim' Whitaker. And locomotive 522.

Confirmed that the unidentified guy on the left is NOT actor Al Taylor.

Got this photo many years ago from Debbie and Tom Bahn. Debbie is Slim Whitaker's great granddaughter. This was among many photos in the collection of Slim's daughter, Leota Whitaker Gandrau. Nothing on the front or back to identify the film title.

Sent out a bunch of e-mails to my contacts in hopes of getting an ID on this photo. No luck on pinning a name on that guy on the left.

From a Nat Levine / Mascot serial ? No luck finding this photo in my copies of "The Vanishing Legion : a History of Mascot Pictures, 1927-1935" by Jon Tuska and Gene Fernett's "Next time drive off the cliff! ... A history of Mascot Pictures".

Internet Movie Database has a smaller version of this image on their Tom London page ... but no film title is listed: https://www.imdb.com/media/rm2126027264/nm0163417

Began searching various websites with info on trains used in films. Got lucky and found the above photo of Canadian Pacific Railway steam locomotive 522 at: http://www.trainweb.org/oldtimetrains/photos/cpr_steam/one.htm

Note the film title mentions - SILENT BARRIERS and a later, longer version titled THE GREAT BARRIER.

The Internet Movie Database has info on SILENT BARRIERS (Gaumont British Prod, 1937). Filmed in Canada, Richard Arlen was the star and the story is about building the Canadian Pacific railroad: https://www.imdb.com/title/tt0028951/

YouTube has a 44 minute documentary on the film. There's mention of Locomotive 522 beginning at around 9 minutes: https://youtu.be/D5BoXGYv3Us?t=579

Revelstoke Museum and Archives in Revelstoke, British Columbia has some photos from the film shoot:

Cast and crew shot - shows Locomotive 522, Tom London, 'Slim' Whitaker, others: https://bcrdh.ca/islandora/object/revelstokeRail%3A1599?solr_nav%5Bid%5D=687d73c9537d822366b4&solr_nav%5Bpage%5D=0&solr_nav%5Boffset%5D=3

Jack Rockwell and others: https://bcrdh.ca/islandora/object/revelstoke%3A25252?solr_nav%5Bid%5D=afefcdbbf7653f177f51&solr_nav%5Bpage%5D=1&solr_nav%5Boffset%5D=5

YouTube has a fair quality version of SILENT BARRIERS to view or download. There's a gazillion people in crowd scenes and portraying railroad construction workers. Running time is about 68 minutes. Go to: https://www.youtube.com/watch?v=Dpw_r6FdcTU

Screen captures from the video: