That graph is NOT the actual voltage, that is the digital signal. Read this, it clearly says there is no negative voltage. The digital signal rides above 0 volts. This is a good explanation.
Here's an excerpt:
"This is not an AC[15] signal, as the rail will have positive voltage or none at any point in time. What is happening is the current is changing direction as it moves from one rail to another. Measuring this with an oscilloscope will display a peak-to-peak signal, which many will claim supports their assertion that the signal is an AC waveform. Since this measurement is made with the track outputs floating, the trace indicates which connection was more positive than the other at a point in time.[16]
Mathematically, the signal would be expressed as X +jY + X − jY, where X is time and Y is the amplitude or voltage.
This method of transmitting data and power in the same signal results in a very robust signalling technique with a high signal to noise ratio while reducing the space charge around the rails and the electrostatic attraction of contaminants. A multifunction decoder can receive data regardless of the orientation of the locomotive, as either rail has data available."
Ted
