Blocks (AKA "branches")
Some modelers with more than 1 locomotive like to make an engine trap- or a siding electrically separated from the main line. To do this, you need another terminal track in the siding. If the track is already laid, then cut through the rail; only one of them. If not, then I recommend using a plastic rail joiner. Run a wire from the rail that was separated (cut or plastic rail joiner) to one of the poles on a SPST switch (available at stores like Radio Shack, for about $.50). Trace the rail with the separation back to the original terminal track. Find which wire feeds this rail, and run a wire from that terminal on the powerpack to the other terminal on the SPST switch. With the switch thrown in one direction, power should reach the siding. In the other position, it should not allow power in, and the locomotive will be trapped in the siding.
Track sections that double back on itself (trains reverse direction while running) are called reversing loops and require special wiring. It is important to make the reversing loop its own electrical block, and wire it as shown in the picture. That is, use a DPDT switch with the centers going to the reversing loop and the feeders X'ed. You should also wire your mainline similarly through a DPDT switch.
As a train approaches the loop, change the DPDT switch for the reversing loop so it is the same as the mainline one. While the locomotive is completely on the reversing loop, switch the DPDT switch for the mainline, to correct the polarities. The train should come seamlessly back to the mainline.