The underside of the tender with the coupling in position before replacing the screw. The spring is not there to act as a spring. It is there to make contact with the base of the weight at one end and with the screw in the coupling at the other. The loco end of this articulated coupling is in contact with the loco chassis, to complete the connection. Note the clear plastic insultaion on the tender axles at the hub of the wheels on the left of the tender (bottom of the picture). This plastic hub isolates the left side wheels from the right. I fitting DCC, the earth arrangement for the connection may be adequate - but you may prefer to add in an extra wire to the loco to be certain.