power loss

i am trying to set up a side track with return to main track with a 022 lionel switch but having trouble losing power as soon as the Locomotive enters the side track. After the engine stops I check the power of the track prior to the switch with a mutimeter and have power to both rails but when checking the power directly in front of the engine both rails are dead. To add to this as soon as I take the engine off the side track the power is back to normal on the side track. I need some help figuring this out. Thanks