As per my message in the
N scale forum thread, your options for running two trains with DC control are somewhat limited.
In order to run trains with DC, you need to create at least as many electrically isolated blocks as you have locos. Ideally you will have at least twice as many (one block to operate in, and one prepared (electrically switched) to receive the loco).
In the diagram below, I have shown the minimum two block. This gives one block for each loco. If the loco (successfully) crosses into the other block, it will be controlled by the other powerpack. This configuration (with insulated joints at the green lines) allows you to have one train running the "mainline", and the other on the switchback/spurs in the centre.
With this very simple blocking (with one powerpack per block, and no other wiring) it is not really possible to have the train run from the top yard all the way to the other end, and then enter the switching area.
Note that there are other wiring configurations that will allow you to run a train right through, but these get progressively more complicated (although not much more) and begin to require electrical switches that control which powerpack is connected to which block. More on that if you are so inclined...
Andrew