Mine has been in place for 35 years as soon as the main benchwork was in place. It’s just a good quality household type on off light switch. One wire of the house current has to pass through all 4 switches. Any one of them flipped off shuts everything down. The wire runs through metal conduit from one box to another under the layout so there’s no way you can confuse it with layout wiring.
This is a timely post. Wanting to get more into operations. I know where they all are. But they are not marked for visitors that may have trouble finding them.
I did a similar continuous string, also using that type of switch, about 25 years ago; at that time it was possible to buy a plastic red faceplate with EMERGENCY heat stamped on the face in black. The store didn't have enough of them in stock, so for the rest of the switches I used plastic white/ivory faceplates painted bright red.
But I did all of this in low voltage DC; I personally would never run 120 around a layout, no matter how well protected. Flipping any any switch down drops out a relay which in turn opens relays on all cab and aux outputs. Restoration not only requires the ES switch to be restored, but a 'reset' pushbutton on the main control panel -- the ES string is the seal in circuit.
Best regards, SZ