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Adam, an e-unit is the part of your train that makes it change direction. On old trains like mine, it's located in the tender. On newer trains like yours, it's probably been replaced by a circuit-board. An e-unit uses electricity to create a magnetic field; the field moves a metal plunger. The plunger rotates a plastic drum that has copper on it and there are other copper pieces that have lil fingers that ride on the drum to complete circuits. If you have an interest in them, let me know and I'll help you find out more about them.
 

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Hi bigtrucker,

I'm just learning the lingo, too. I think that Lionel made a few version of it's e-units ... a four-position one (fwd / neutral / reverse / neutral ... then repeat), and a two-position one (fwd / reverse ... then repeat). My little #249 loco has the two-position one mounted on the rear of the engine. The e-units usually have a little lever that you can position to engage the e-unit auto-direction ability, or turn that funcion off, if desired. My understanding is the the e-unit solenoid senses a short-duration drop in track voltage ... this pushes the solenoid plunger up/down, which, in turn, mechanically moves a little rotating toggle that flips the direction of the wire leads to change the current direction ... and hence, the train direction. Pretty ingenius thinking for post-war, pre-diode and computer chip technology!
 

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Hi bigtrucker,

I'm just learning the lingo, too. I think that Lionel made a few version of it's e-units ... a four-position one (fwd / neutral / reverse / neutral ... then repeat), and a two-position one (fwd / reverse ... then repeat). My little #249 loco has the two-position one mounted on the rear of the engine. The e-units usually have a little lever that you can position to engage the e-unit auto-direction ability, or turn that funcion off, if desired. My understanding is the the e-unit solenoid senses a short-duration drop in track voltage ... this pushes the solenoid plunger up/down, which, in turn, mechanically moves a little rotating toggle that flips the direction of the wire leads to change the current direction ... and hence, the train direction. Pretty ingenius thinking for post-war, pre-diode and computer chip technology!
It's the same thing for AF, too. If you'll allow me to tweak it a bit, here's my version. An e-unit has 4 basic components: 1. a coil, 2. A plunger that is moved by the coil, 3. A drum with a patterned copper overlay, and 4. The fingers (2 sets). It works this way. When you turn on the transformer, power flows to the coil (usually mounted in the tender) and the plunger is jerked up and held there. As long as power is not interrupted, nothing happens. However, at some point, you shut off the transformer, derail, or hit a dead point in your track. Once this happens, the power fails and the plunger falls.
When the plunger falls, it rotates the drum one click. This changes the setting for the drum and when you power up again, it's at a different setting. Sequence is either two-position or four-position, depending on your e-unit. If it's a two-position, every click on the drum is from foward to reverse or reverse to forward---two positions.
For a 4-position e-unit, the sequence is forward-neutral-reverse-neutral. This is much gentler on the engine, as it keeps you from going at full-tilt forward and suddenly slamming into reverse.

Now, back to drums and fingers. The drum has a copper sheath that has cut-outs. The fingers are metal contacts that ride on the drum. For each click as the drum rotates, different fingers come into contact with the copper sheath and complete connections. 1/4 of those positions make the engine go forward. 1/4 make it reverse, and 1/2 make it do nothing except smile at you while powered cars do neat things like dump their loads or add/lose passengers while the train rests, motionless. As TJ pointed out, it's a pretty elegant way of using pre-WWII electro-mechanical technology to do tasks that are now computerized.
 
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