Union Pacific Rocket Train SSCOG

Each of those cylindrical cars contains an individual segment of a solid fuel rocket booster which contains a single "Grain" of the fuel. The grain has a hole through the middle and when stacked form a set which when ignited from the top burns over the entire surface of each grain; inward from the outside, outward from the inside and from both ends of EACH Grain towards their middle.

The purpose of this is to insure that there is the maximum amount of fuel burning without over pressurizing the housing with the result being a CATO of the motor structure.

It why they call it "Rocket science".