CW,
When Gilbert first moved the e-unit from the locomotive to the tender, they used a 4-conductor wire to a jack panel at the back of the locomotive. This allowed the tender to be disconnected from the locomotive. This resulted in the headlight being off and no current to the smoke unit when the locomotive was in neutral. When the locomotive first began to move, it took the smoke unit a few seconds to heat up instead of smoking immediately.
To correct this problem, they added a fifth wire that provided a direct hot lead from the track to the headlight and smoke unit. They still used the jack panel, however, which didn't make sense because you still have to un-solder the fifth wire at the a jack panel if you want to disconnect the tender. In 1955-1956, they ditched the jack panel and went to the direct wiring that you have.
