MTH - RailKing Freight Cars - 3-Dome Tank Car - O Scale 3 Rail 

Three-dome tank cars were actually three complete tanks on a single car body. Each tank had its own end bulkheads and expansion dome. Since older tank cars were filled to the top of the tank to prevent fluids from sloshing around during shipment (which could make the car unsteady), the domes provided space for the cargo to expand when the sun heated the tank. Because the additional bulkheads and domes added weight, multiple-dome cars had a smaller capacity than single-dome cars.