A runaway loco is any working DCC locomotive that has suddenly taken off on its own power and/or you no longer have any control over it.
(This runaway problem is not to be confused with the problem with using 128 Speed Step mode with a decoder that does not support 128 speed step mode.)
This section talks about the what how and why's of locomotive runaways.
1) WHAT ARE THE CAUSES OF ANY LOCOMOTIVE RUNAWAY?
All Runaways fall into 2 high level categories.
A) User Created
i) ON POWER UP: The throttle speed was not turned down to zero when power was shutdown. So the trains takes off when power is restored.
ii) WHILE RUNNING: Two throttle are somehow assigned to the same engine and they are fighting for control. One has one speed setting that is different than the other.
B) Electrically created.
Loco takes off on power up, being placed on the track or part of a derailment.
i) OTHER POWER SOURCES: Something else is connected to the track injecting power. Could be a DC power pack or some constant lighting unit that was used with DC operation. Decoder believes it is another power source and engine takes off.
ii) NOISY CONNECTION TO DCC POWER: The act of placing a engine on the track will involve random connection of power between the rail and the wheels. The corrupted DCC power is interpreted as DC power and the engine takes off
C) Derailment
Loco is programming is corrupted or damaged electrically during a derailment.
Short circuit involving a derailment are never clean short circuits. They are a series of short and open that occur in a series due to the random nature of the wheel to rail contact during the mechanical shock and vibration created by derailment event.
2) ELECTRICALLY WHAT ARE THE CAUSES OF LOCOMOTIVE RUNAWAY
All runaway events under DCC operation are directly tied to track power when:
1) alternative power is present (DC for example). This is especially possible with layouts that were converted from DC to DCC but had some advanced features to the DC layout like a constant lighting unit connected to the track. Or it can be a DC power pack left connected to DCC power track that is corrupting the DCC signal and/or introducing DC
2) corruption of the DCC power in the form of unrecognizable DCC commands. See Snubber/RC Filter
3) hazardous voltages in the form of voltage spikes AS SEEN BY THE DECODER. See Snubber/RC Filter