These bars are showing the amount of commodities that the freight cars, passengers, and fuel if enabled for the locomotives, that is being carried.
If you choose not to see this in Driver, you can turn this off in either the Session editor, or Surveyor by clicking on the PC display icon, just to the right of the wrench and screw driver icon, and then clicking on Commodity Overlays. Under there are options including set to Always Hidden, or Always Visible. Pick which one you want.
