Answer:
Standard Oil controls the docks and railroads so independent dealers cannot ship oil.
Step-by-step explanation:
Standard Oil was a monopoly that was also vertically integrated with key parts of its supply chain like docks and railroads.
In the excerpt, the independent dealer is complaining about the control that Standard Oil has, not only over the oil wells, but also of the transportation means that are used to ship the oil. This is why the independent dealer confeses that he has to sneak out the barrels.