The tracks or rails of the subway are actually supposed to flex to some degree as the trains run over them. They bow and bend slightly in response to the stress placed on them.
If they were to ridged, they would literally snap when rolled over. So they need to bend somewhat.
Well, the tracks at one of my subway stop bend a LOT! The bend down easily 6 or 8 inches when the train passes. The whole track dips including the wooden stretchers. The tracks are laid over gravel stones instead of concrete, so there’s space under neath them.
Then the train comes, they bend. There’s also a rail joint at that same spot. So when the train goes by and hits the joint, there’s a very loud bang.
I tried to get this video to show it. You can somewhat see the rail move if you look closely. In person, you can really see it.