The info above is correct, but here's a couple of photos which may help...


Take care when disengaging the tabs at the rear, as they can easily break, which will then leave the rear of the tender unfastened to the underframe.
EDIT: While all of my locomotives are strictly DC-only, it seems to me that the location of that screw might be in the way of adding a decoder for DCC. I'm currently modifiying a Bachmann Long USRA tender for a friend, who asked me to install the stuff needed for a so-called "dead-rail" installation, where all power comes from a battery.
While it's a tight squeeze, I have most of the equipment installed, which includes the battery, plus a DCC decoder, along with the necessary switch for RUN/OFF/CHARGE, and a plug-in port for the battery charger. All that needs to be added are resistors for the lights and wires to connect to the motor, headlight, and back-up light....

To fit all that stuff into the tender shell, I cut off the peg which accepts the screw which holds the tender body to the underframe, and added some strip styrene to the water-legs of the tender, then drilled and tapped them to accept 1-72 screws. Suitable holes were then drilled into the tender's underframe.
Wayne