It's a bit sad that many things previously accomplished by rail has moved to boring trucks. Nowadays the clay for Haga Tegelbruk (in english "tilery", which you probably can hear buzzing and beeping in the background) comes by truck from a few kilometers away. The place you are standing on was probably a loading station for clay. The tilery has been around for a few hundred years.
More reading: Haga tegelbruk and Haga slott's wikipedia page
In case you plan on taking the caches in Bredsand on the other side of the river: Good luck! Even though they seem to be just a minimum distance away from where you are standing, the trip there will be 10-12km, if you don't swim or take a boat. The nearest bridge is in the middle of Enköping!