First, you imply not only a run away train, but also run away smoke unit too...burning furiously and dry. I would add smoke fluid and if it heavily smoking you may have a Boiler board issue causing some of this beside a possible tach reader issue. BUT smoke does matter as a bad (electronically noise smoke fan) can cause issues with speed control. Additionally a ground on the motor leads can also cause a run away motor. As can a ground on the smoke element return wire causing excessive smoke.
So now that we know what dcs system you are using, and you seem to have a run away smoke unit also; you do need to go in and unplug the wires that go to the smoke fan and smoke heater. Now some RK are direct wired and you might have to desolder and isolate the exposed ends to ensure they do not ground out to chassis. If that returns control to the engine for speed, you need to examine the boiler board out of the plastic holder. You would see damaged components if a fet is shorted.
Frankly you have a more significant issue and I would recommend you send this to a tech. Engine wiring needs to be validated also.
Having said that, have you run this just conventional no Remote cdr. Do you power remote commander with brick, NOT the Z-750 controller? If both test have similiar results you need the engine checked. You can put brand new boards in and blow them immediately if the intial cause is not rectified. G
