Jean Gabin really likes trains. Jean Renoir also really likes trains. They wanted to make a train movie, and any train movie would do. So why not one that also includes murrrrrrrrder?
La Bete Humaine has a lot of bad psychology and therefore some bad social commentary. It also misses a theme from the original novel that it seems like Renoir and Gabin — who had just finished The Grand Illusion — should have leaned into but instead ignored. But it does have trains! Lots and lots of trains!