Physical vs Logical

In computer systems, particularly in networking, there is this concept of physical versus logical. Some people have difficulty understanding the difference. This is a picture I saw the other day at a subway station. It's a diagram of the various lines that run through the tracks. As you can see, there are several train lines that run on the same physical track between the Montgomery and Van Ness stations. Then after station Van Ness, the N line goes off into another direction running on its own dedicated physical track. Likewise, the J line does the same. The remaining lines continue to run on the same track all the way to the West Portal station.

If this were a computer network, the train track is the physical aspect of the network while the various lines are the logical aspect. You can have one physical network cable that carries multiple network signals using various protocols. If this were a computer, then you have one physical computer can can do many things like playing music, accessing the Internet, and so on.