As far as I know horse racing had inside rails really early. So I don't think it's a horse track.
I do think it's a board track for bicycles or early motorcycles. Those tracks often had a low curb at the bottom just like you can see at the right, and on the left side past the people, you can see the beginning of the banked curve and a pretty typical railing/fence they had at the top of the banked curves.
https://ephemeralnewyork.wordpress.c...-racing-1920s/
The bathrobe guy may be a rider.
But the gangster idea wouldn't be a stretch, Capone was apparently a huge cycling fan. And Cycling was a big sport pretty much everywhere including Italy.