"The success of the Densmore Tank Car revolutionized the transportation of oil by railway, leading competitors to copy the car’s simple design and to improve upon it. Over the next several decades, many improvements were made, including metal construction, a single-tank design, and expansion domes that prevented leakage. By 1868, a boiler-type tank car was in use. These cylindrical horizontal tanks could carry 80 to 100 barrels of oil each. The new tank cars were not top-heavy and were made of metal, so they did not leak oil." (Quoted from paper)