Test Light

One of the most useful diagnostic tools is a simple test light. The test light I use most often has an alligator clip on one end and a light bulb and long sharp metal probe on the other. It looks similar to the ones here: http://www.harborfreight.com/3-piece-circuit-tester-set-94130.html (It likely came out of that set.)

The test light lights when a circuit has been completed. That usually occurs when one side of the light is connected to Rail A and the other to Rail B. The test light requires voltage to be present for operation, but with DCC this is no problem. With DC, power to the block must be on and the throttle set high enough to turn on the light. If either side is not connected to the rail or connected to rails of the same polarity, the light will not turn on.

Locomotives run under the same conditions that make a test light light. Just as a locomotive will move with the rails at any polarity, the test light will light with the rails at any polarity. Conversely, the test light will not light under conditions where the locomotive will not run, so it's safe to assume that if the light is lit a locomotive will run. (A DCC locomotive at speed step 0 is still technically running. The decoder is receiving power and interpreting commands.)

The test light works according to this truth table: (This doesn't represent all the options, just the important ones.)

Probe A | Probe B       | Result

--------------------------------

Rail A  | Not connected | Dark

Rail A  | Rail B        | Lit

Rail A  | Rail A        | Dark

Note where the probe lights when connected to different rails, but does not light when connected to the same rail.

Armed with this information, consider the following scenario: Trains can run on a section of straight track until they reach a specific point, then stop dead. How do you diagnose where this happens with a test light, and determine what rail the issue is in?