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.
These cars were popular with customers who needed several varieties of a single product, such as different grades of lube oil or gasoline. They were also used for customers who did not need a full car of product. While smaller-volume shipments have largely gone over to truckers today, Union Tank Car built new multiple-compartment cars as recently as 2010. They're harder to spot in today's trains, however, because modern tank cars do not have expansion domes.
High quality, traditionally sized RailKing Freight Cars provide detailed bodies and colorful paint schemes for the O Gauge railroader. MTH makes an enormous variety of RailKing Freight Cars, including many different car types and roadnames. No matter what era or part of the country you are modeling, RailKing is sure to have something for you.