The below simplified schematic illustrates how I troubleshooted the location of the short circuit (marked with the red S).



In this set-up the track acted as a long wire, which connected the bulb to the power supply. And since there was a short circuit somewhere on the track, the circuit was closed and the bulb was lit.

By measuring the voltage drop between two nearby points on the track, I was able to dertermine which direction the current was flowing in each point, starting from the point where the power supply +-lead was connected to the track. Even over a couple of track pieces there was a voltage drop of about 2-4 mV, which was measurable. But an accurate reading was not important, just whether it was negative or positive value. This indicated the direction. This was the reason to use DC power instead of the regular AC supply.

So lets take this example. I first measure the voltage drop between points 1 and 2. It shows a small positive value, something around +3mV. An exact reading is not important, it still shows that the current is flowing from point 1 to point 2, in that direction. Then I repeat the measurement between the next points 2 and 3, 3 and 4, and 4 and 5, and get the same results. The current is still flowing forward. But when I measure the same between points 6 and 7, I get a negative reading, so somewhere between points 5 and 6 the current changed the direction. That is the point of the short circuit.

The same can be repeated from the opposite direction, starting from points 11 and 10. You get the same readings until the point 6, but when you proceed to measure between points 5 and 4, the sign of the reading changes to the opposite again.

I hope this clarified the method I used.