Steam & Excursion > How Does This Signal Pickup Work?

Obviously each "thing" is a coil which picks up the magnetic field from the rails for ATC or cab signals or whatever.  My question is how is there anything to pick up at that location, under a steam locomotive tender.  Wouldn't the wheels fore and aft short out the signal before it ever got to this point?  I thought these things have to be positioned in front of the first axle. I'm curious.

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Disclaimer: I'm an electrical engineer who isn't at all familiar with the details of RR magnetic pickup signal systems. However...... Being an engineer, that won't stop me from speculating and hoping that it uncloaks someone to correct me!

What locomotive is this? Historically, mag pickup systems like this were specific to the RR signal system, there isn't a standard I'm aware of.

The two applications I'm aware of are ATS type applications, where you have an on/off type signal at a specific control point, and cab signaling such as was found on the Pennsylvania lines.

In the photo, I'm seeing 4 transducers, two set at about the rail, and two inboard of the rail.

Position of the wheels, etc. is more or less irrelevant. These coils are going to each produce a change of signal when the ambient magnetic field changes around them. The tender, wheels, and rails are all going to remain more-or-less the same, so the signal from each of these transducers is going to remain the same until something in the magnetic environment changes.

Such as:
- Switch guard rails, frogs, points (a source of signal noise that has to be ignored)
- A device between the rails that is producing a strong magnetic field (part of the signalling system)

With two transducers placed between the rails, that gives the opportunity to place a pair of transducers on the track structure. That gives you a 2 bit command signal: indications 0, 1, 2 and 3.

So, if neither of the in-track transmitters is powered, the transducers don't sense anything different and you get no change in signal.

If one track transmitter is powered, the transducer closest to that transmitter produces a signal that's stronger than it had been. Some associated relay logic would 'pick up' that change and lock it in. (This is 19th century computer memory, and the same technology used in any signal system in the bungalow.)

Now, as I said when I started this reply, I know zero about the actual implementation you took a picture of, I just know from an engineering standpoint what you can do with a transducer arrangement like this and I'm basically speculating. I'm hoping someone with better historical knowledge can chime in here.

SRK

Steve Krause
Chillicothe, IL

I believe this to be a continuous system, not the intermittent signal like ATS which most of us are familiar with that is active when the pickup aligns momentarily with the trackside inductive device.

This is on an engine at IRM but I've forgotten which one; I am inclined to say it was the GTW 4-8-4 as that engine has a Vanderbilt tender.

Shown here is the pickup under IC Paducah-rebuilt Geep 8408 on display at Homewood, IL.  Under that mud buildup would be coil pickups and the placement was ahead of the first axle.



Edited 1 time(s). Last edit at 07/05/15 09:13 by filmteknik.

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That is the ATC, CCS or other system pickup bar that transduces the signal through decoders to the signals in the locomotive cab.

Bob Krieger
Cheyenne, WY

With coninuous systems, the pick-ups must be ahead of the leading wheels in the direction of movement because the signal to be received is in the track circuit in front of the train, and the signal is shorted circuited by the leading wheels. The OP's pic is of some variety of intermittent inductive set-up which uses trackside magnets, which aren't dependent on track circuits to send the signal.

Yeah that makes sense.  Not something I'm familiar with but I'm correct that it can't be something through the rails.

Hello filmteknik,

Would you be so kind as to identify what tender/loco combination it is that you have pictured here.

The GTW had nothing like this that I ever saw.

Just wondering.

Thanks!