Jay says:

Much depends upon how the tender is made. Some tenders pick up both rails of power, while some only pick up one rail, while the loco picks up the other. You also should make sure that the loco’s motor does not draw too much current, I usually install a can motor, and remove older open frame motors, this also depends upon the current rating of the decoder you are using. It sounds like your tender picks up on rail. So, you will run the tender’s pickup to the decoder’s input, and the loco’s pickup to the other decoder’s power input. Your instructions should tell you left rail power and right rail power. You should use a micro JST connector between the loco & tender, and keep the wire gauge light to prevent too much stress between the two. You will then need to run the 2 wires to the motor from the decoder inside of the tender. Also two wires for the headlight, so now you will have 5 wires from tender to loco. You will need a 5 conductor connector set if you ever want to separate loco from tender or you could leave them permanently paired…