You will have to run a pair of wires from each circuit breaker to the district it is protecting. Any track feeders must only be connected to it's respective buss pair. Both rails must have insulated joiners where one district ends and the next one begins.
According to the PM42 manual, page 4, "tracks should be double gapped between adjacent power districts."
John
The Dames Point Industrial Railroad
http:\\dpirr.blogspot.com
Craig North Carolina