The Innotek SD-3125 Contain 'n' Train electric dog fence
collar is perfect for those looking to make their SD-3000 or SD-3100 into a 2 dog contain 'n' train system. Features:Additional Contain 'n' Train replacement collarWater resistantReplaceable 6 volt batteryAdjustable nylon collarCompatible With:SD-3000SD-3100CT-400ACKC-25W
NOTE: Only the SD-3100 can operate as a 2-Dog Contain 'n' Train System. For all other systems this is a standard replacement collar.