It is important that the relay bracket is completely insulated from the frame. The use of the insulation material and the shoulder washers should achieve this.
One note about the different relays: some, like the one pictured, have a wire that connects to ground via a solder lug. This is fastened down by one of the relay mounting screws. Other relays do not have this wire. The ones that do not have the wire are designed for the steam whistles in tenders. The whistles are powered by track current. The horns are powered by a dry cell. That's why the relays for the horns must be insulated from the frames. The relays for whistles are not insulated and therefore do not need the extra connection to the ground.
The diagram on the left shows how the components are assembled for the horn. The diagram on the right depicts the wiring.