Dieselpunk is a genre of retrofuturism in art, literature, fashion, design, film, and music, that blends the aesthetics of the 1920s to 1940s (the diesel age) into modern lifestyle, or follows an alternative history approach in art projects in the sense of 'a past the never was', often in conjunction with science fiction elements.
The following picture, which was made by Midjourney as an illustration for a forum article, shows a typical dieselpunk scene:
One of the most famous online dieselpunk sites is the award-winning Metropius art project by Australian film maker Dan Macarthur. It also runs a Discord server for all folks interested in dieselpunk.
Dieselpunk City is a series of film noir inspired pictures created by Midjourney showing street and railroad scenes in a fictitious dieselpunk city that resembles the atmosphere of a 1930s Chicago. Originally, only 12 pictures were planned. Surprised by the overwhelming attention that series attracted on some art platforms with selected pictures even going viral, more pictures will follow.
For the full series, see this album on Flickr or this gallery on Deviantart.