Since you are trying to convert the Lionel 1121 switch that doesn't have the non-derailing feature into a switch that does have the non-derailing feature, then it doesn't matter which outside rail is used for the insulated rail / insulating pins. Don't forget that you need insulating pins at both ends of the insulated rail. And you are correct in using the "side of the switch with 2 sets of 3 rails."
It does not matter.
You don't even need to put the insulated track section(and pins) immediately adjacent to the switch, but a section or two away works well and gives the switch time to throw completely, especially for diesels with rubber traction tires on the lead truck. It could even be on the other side of the layout for fancy train routing effects, or you can have more than one trigger section. You could even have a train on a different track throw the switch if you can be sure it's done when there's no train on the switch.
With 1122/1122E switches, you do need to insulate the trigger rails at the switch or an adjacent insulated rail trigger section, or the switch motors will be continuously energized.
Rob
The short inside curved and straight rails get the fiber pins.
Working with 1121 switches I am setting them up for non-derailing using insulated track. Question is does it matter which outside rails of the switch is used to insert the insulating pins? I know the one side of the switch with 2 sets of 3 rails is used in doing this, but which rails should get the insulating pins?
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