Some years ago (mid 80's) I was responsible for cleaning at 20 Tesco stores in the South West.
At the time we had a guy visiting with a pressure washer jetting off the wheels of supermarket trollies at 50 pence a trolley!!!
It was the trollies (including the 'shelf stacking trollies') that left black lines around the floor of the store.
When you consider how many there are at each store and the way they are stowed inside each other it doesn't take a brain surgeon to work out that there's good money to be made there.
Does anyone still do this?