What is it and how do I find a DPDT?
It's an electrical switch; double pole - double throw (DPDT).
Here is a schematic of a DPDT:
More typically, they look like this:
The DPDT is very common to model railroading and found at just about any store that sells electronics. It comes in a variety of arrangements, with the most common being a DPDT Center-Off switch.
In the early days of Direct Current model railroading, reverse loops had to be controlled by hand, and the DPDT was perfect for the job. The DPDT was wired in such a way that power was fed into the switch from the transformer and then the track polarity could be changed by throwing the DPDT. While the train was moving through the reverse loop, the track polarity of the main line was changed with a second DPDT:
The reverse switch inside the DC train transformer was also a DPDT, wired in the same fashion but could not be used since the train was in motion. So, the main line and the reverse loop had to be controlled in this manner.
Now, of course, DCC offers autoreversing, either with the PM42 or the AR1, so the DPDT is no longer used in this way. However, there are still uses for the DPDT, such as wiring a programming track that is part of the railroad:
The programming track is isolated from the rest of the railroad, and whether the track is connected to the Rail A and Rail B connections or to the programming connections is switched with a DPDT.