The rubber grommets used in the old motor mountings were to minimize any vibration and noises from the motor or the drive line. The brush contacts for the motor and also its field are already insulated from the motor's frame.
Newer 'can' type motors run much more quietly with less 'cogging' of the armature than the old motors did. So they usually do not require that sort of isolation from the locomotive frame. Still, if the modern motors can also be isolated that way, they could be even quieter in operation.
Ed Bommer
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