Anybody have a photo showing the surge protection diode on a MTH Z controller circuit board. or tell me location.
Do you mean the TVS? Photo below shows a Transient Voltage Suppressor (TVS) diode connected between the Hot (Green) and Common(White) on a plate-jumpered distribution strip with capacity for routing protected railpower from a PoHo 180 to various scattered soldered connections in a single power district.
This particular T-strip feeds from 5 protected hot and common T-strip screw heads to five different 37" lengths of Gargraves flex track in an oval. Other T-strips at different locations on the layout also suppl railpower and protect the fragile locomotive components from voltage spikes while routing railpower to other center and outside rails.
The transformer is protected from current surges by the circuit breaker. A voltage spike does not trip a breaker/fuse as does overcurrent. The TVS is necessary protection of circuit board components.