For me, the appeal of unit trains is that they are so realistic. Real freight trains often have many of the same train car, such as oil tanker cars, hoppers, boxcars, refrigerator cars, gondolas, etc. Seeing a long unit freight train can be breathtaking.
It can be challenging for an O Gauge model railroader to assemble a long unit freight train. Some of the freight cars are expensive.
If affordability is desired, I recommend starting with some Lionel Postwar Sunoco oil tanker cars. That was my 1st unit train. These oil tanker cars do nothing, and can be purchased with some rust (very prototypical) for as little as $5 per car. I look for ones that have good couplers and that have wheels that are in decent shape. A cleaning with warm water and ivory soap plus one drop of light oil on the inside and outside of each wheel goes a long way to forming a nice unit train of 8 to 12 cars or more. If there is crud on the wheels, I scrape it off with the blade of a Swiss Army knife.
Although I do not have this yet, another affordable unit train would have 10 or more Postwar Lionel gondolas. They also do nothing and can be purchased for a very modest price (between $5 and $10 per car) at train shows. What is stated above regarding oil tankers applies to gondolas.
Arnold
