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Offline xxup  
#1 Posted : 03 June 2019 23:54:31(UTC)
xxup

Australia   
Joined: 15/03/2003(UTC)
Posts: 9,458
Location: Australia
Some railcars like the Marklin 3471 Ram TEE have built in circuitry to switch over the live pickup shoe with a change in direction. That is, when the railcar is travelling forward the front pickup shoe is used for current flow, while the rear shoe is electrically disconnected. When the railcar runs in the reverse direction, the rear pickup shoe is used and the front is electrically disabled.

Why is this so? Why not have both shoes electrically connected?

Is it to do with parking a railcar across a booster boundary?
Adrian
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Offline kiwiAlan  
#2 Posted : 04 June 2019 00:13:07(UTC)
kiwiAlan

United Kingdom   
Joined: 23/07/2014(UTC)
Posts: 8,082
Location: ENGLAND, Didcot
Originally Posted by: xxup Go to Quoted Post
Some railcars like the Marklin 3471 Ram TEE have built in circuitry to switch over the live pickup shoe with a change in direction. That is, when the railcar is travelling forward the front pickup shoe is used for current flow, while the rear shoe is electrically disconnected. When the railcar runs in the reverse direction, the rear pickup shoe is used and the front is electrically disabled.

Why is this so? Why not have both shoes electrically connected?

Is it to do with parking a railcar across a booster boundary?


Historically it is to do with stopping at a signal using the switch in the signal under analogue power. You want the pickup shoe at the front of the train so that when it reaches the dead portion of track the front of the train has not passed the signal. If the train is operating from a pickup shoe at the rear of the train (in the running direction) then unless you have very long dead sections at a signal most of the train would be past the signal before it stops - not exactly prototypical, and possibly causing an accident if the signal is protecting a train that is (say) stopped at a station.

But if you have long dead sections then if the train approaching a signal has the pickup shoe at the front then it stops a long way before the signal. So the solution on a long train that can run either direction is to have the switching pickup shoe.

This has carried over into digital because of the various methods of stopping the train at a signal by using asymmetrical signal or DC switching. Again the stopping point relies on the pickup shoe being at the front of the train, so there are various systems using relays (available from Marklin and ESU, and possibly others) that are operated by the decoder depending on requested direction.

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Offline xxup  
#3 Posted : 04 June 2019 00:36:38(UTC)
xxup

Australia   
Joined: 15/03/2003(UTC)
Posts: 9,458
Location: Australia
Thank you.. That makes sense to me..
Adrian
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Offline mike c  
#4 Posted : 04 June 2019 01:02:33(UTC)
mike c

Canada   
Joined: 28/11/2007(UTC)
Posts: 7,880
Location: Montreal, QC
An additional benefit of the dual sliders was that the train lighting and head/taillights were powered from the trailing slider, so that the trainset would stop when it entered a signal zone, but the lights would remain on while the train was stopped. This was accomplished by having one relay for direction and a second relay which would select which power source was used for the motor and for the lights.
Technological changes allowed the change from the 3 wire connection of the original 3070 to the 2 wire connection of the 3071 and the delta/digital connections of the 3471/39700 which required more connections because the return was also connected through the decoder rather than through the car bodies and wheelsets. I do not know if the lights are still powered by the trailing slider on the delta/digital models.

Regards

Mike C
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