Simply set the minimum amount that each train car can carry in the red combinators, and let the station do the rest.
This station makes sure each section has at least one train cars worth of materials buffered in each section. If any section does not have enough materials, then all other (full) sections automatically stop their input until that section is has reached the minimum amount.
This is better than (or at least supplements) balancers. A balancer always tries to give the same amount of materials to each lane, but what if one section being drawn from more than the others? The buffer chests mean all lanes continue to flow, but that one section stays starved.
Example use case: Stations become uneven any time a train is drawn from unevenly, or mixed length trains are being filled by the same station. The "proper" solution is to never let those two scenarios happen, but this is often unrealistic.