# Question - reverse loop MECHANICS of turnouts



## spacomp92653 (Jul 5, 2018)

So I've watched several videos on reversing loops, both DC and DCC and the electrical part

is very clear, but I can not find mention of turnouts and how they handle the train going through

in both directions without any switching taking place. I'm a rookie, can you tell? I'm guessing the

train runs around the loop, then the weight of the locomotive entering the turnout moves the spring loaded

points momentarily while is passes through, is this correct?

Can any brand of turnout in any scale or gauge work well on a reversing loop?

If a train is backing through a loop and the first car to hit the turnout is very light in weight, can that

cause a problem?


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## timlange3 (Jan 16, 2013)

Prototypes can use spring loaded switches (the points actually) as even the weight of a small empty car is several tons and will move the points. Models on the other hand will have difficulties getting the spring strong enough to move the points and yet allow a light car move them. Best to throw the turnout by hand. Since you may also need to throw an electrical switch to set the correct polarity for your route you may be able to combine the mechanical switch for the points with the electrical switch. This way the route and polarity are set correctly. There are DCC electronics that will change the polarity for you so all you need is to set the route.


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## JeffHurl (Apr 22, 2021)

Yeah, I think it's a given that, for model railroads, we have to actuate the turnout to accommodate the entry/exit from a reverse loop. I'm sure there's someone out there who is an electrical engineer who could devise something to automatically detect that a train is about to enter from one of the frog routes of a turnout, and switch the points automatically. But for me, half the fun is anticipating what points need thrown to successfully navigate my layout.


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## spacomp92653 (Jul 5, 2018)

wow, ok. Not one of the videos I've watched on reversing loops showed the operator having to throw the switch every time. They seem to let the train run unattended and be hands off.

That's the visual at least on the ones I've watched. They go into detail on the electronics and say nothing about the mechanics at all.

So I would definitely not do reversing loops if they require that much attention to throw the switch for every pass through.


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## DonR (Oct 18, 2012)

In actual rail and streetcar operations some turnout
points are spring loaded. This is common on single
track mains that have a 'passing' siding where a train
can wait for another train coming from the opposite
direction. The points are alwaya set for 'divert'.
No need for conductor to throw the points, however,
the loco flanges push the rails aside.

In another thread there was extensive discussion on the
topic of automatic turnout throws. It can be done, but the
complexity of the circuits depends on the actual track
layout and whether turnout motor is Tortoise 'stall motor'
or 'twin coil'. But, in general,
it is one of the model train operator's duties to see that
all turnouts are set to pass the train thru without derail.

Don


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## spacomp92653 (Jul 5, 2018)

To be clear, I was referring to model railroads, not prototype or 'actual' full scale operations.

And wow, the videos make it look so easy to install the electronics and show simply smooth sailing afterwards.

Kinda crazy not one of them mentions keeping your finger on the turnout switch ready to throw it the moment the train 

passes through and before it completes the loop.


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## DonR (Oct 18, 2012)

As I mentioned, there is a difference
in control circuitry for twin coil or
Tortoise type turnout motors, so if you
go ahead with this be sure to note what
the instructions say about how to wire
each of those two turnout motor types
for automatic throw. Note also, that
twin coil motors can use either AC or
DC, but Tortoise and Kato single coil 
requires DC current.

Don


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## traction fan (Oct 5, 2014)

spacomp92653 said:


> wow, ok. Not one of the videos I've watched on reversing loops showed the operator having to throw the switch every time. They seem to let the train run unattended and be hands off.
> 
> That's the visual at least on the ones I've watched. They go into detail on the electronics and say nothing about the mechanics at all.
> 
> So I would definitely not do reversing loops if they require that much attention to throw the switch for every pass through.


spacomp;

It is quite possible to automate the process of throwing the turnout at a reverse loop. You will need a photosensor in at least one, preferably both, ends of the loop where the tracks come into the turnout. When the sensor is activated it throws the turnout to the correct position for the track that particular sensor is monitoring. 
There are other types of sensors that can detect a locomotive, or lighted car. The advantage of photosensors is that they can detect any car that produces a shadow on the sensor. The car does not need to be drawing current, like a loco or lighted car, it doesn't need to be any particular weight, and if you position the sensors properly, even a low flat car will trigger it. There are commercial photosensors made for model railroad use. IRDOT is one brand name. 

Traction Fan


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## spacomp92653 (Jul 5, 2018)

Don R, thanks. Do you have a favorite brand of reverse loop circuitry for DCC? and for DC?


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## cv_acr (Oct 28, 2011)

spacomp92653 said:


> And wow, the videos make it look so easy to install the electronics and show simply smooth sailing afterwards.
> 
> Kinda crazy not one of them mentions keeping your finger on the turnout switch ready to throw it the moment the train


Probably because wiring the track to handle the electrical polarity issues and automating the function of the turnout are two VERY completely different topics. And most of the time, there may be a certain assumption that you're directly controlling your trains; setting up a wholly hands-off automated setup is a bigger topic.

While I haven't set up anything like this, as I'm a more "hands-on" RR operator, there are solutions out there using a photodetector and a relay that will automatically throw a switch as a train approaches it. This is completely independent of actually wiring the reverse loop for polarity issues, however if this is all combined with a simple loop, you could drive the loop polarity change from a relay so it changes with the turnout position.


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## cv_acr (Oct 28, 2011)

spacomp92653 said:


> Don R, thanks. Do you have a favorite brand of reverse loop circuitry for DCC? and for DC?


DCC "Auto Reversers" like a Digitrax AR-1 handle the electrical part of switching the track polarity or phase quite nicely. I don't think these auto-reversers necessarily work with DC the same.

Or you can drive the polarity toggle with the same switch/relay that throws the switch, if you have enough relay contacts.

But has nothing to do with throwing a turnout automatically.


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## DonR (Oct 18, 2012)

I don't have any 'favorite' control circuity for automatic throwing
turnout points by loco. I preferred actual manual push button
control. For yards and routing through a complex track
arrangement there is a very simple diode matrix. It throws
all points in a route with touch of one button. Works only
with DC twin coil turnout motors.

The first thing you'll need is 'detection' of the loco. Photo electric
or similar device would be most available. The next concern is
the type of turnout motor. That would determine whether to use
low voltage DC or AC...Twin coils can use either...Tortoise and
Kato single coil require DC. You would need continuous voltage
feed for Tortoise motors and momentary puls for twin coil or Kato
but Kato also would require ability to reverse polarity which controls
point throw.

To throw the points of a turnout that creates a reverse loop a
detector and circuitry for the turnout motor type is all you need.
There are several detection devices available commercially.

To get more specific we would need to know the make and
model turnout you wanted to be auto thrown...and the type of turnout motor. 

However, my curious mind wonders...could we isolate 1 inch of
one rail and use that as a detector? (loco wheels close circuit,
throwing points) 
We'd have to consider that track
changes polarity. Anyone got any thoughts on that?

Don


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## traction fan (Oct 5, 2014)

DonR said:


> I don't have any 'favorite' control circuity for automatic throwing
> turnout points by loco. I preferred actual manual push button
> control. For yards and routing through a complex track
> arrangement there is a very simple diode matrix. It throws
> ...


Don;

It might work, assuming all wheel electrical pickup and enough wheels on all the locomotives.
The 3-rail modelers, and 1:1 railroads, do this all the time. In both those cases, the two wheels on the outside running rails are permanently shorted by their common axle. The Lionel fans can rely on the center rail to provide consistent power, and a full-size diesel doesn't need to pull its motive power from the rails it rides on. It generates its own electricity internally.

The polarity change for the track can be accomplished "old school" with relay contacts activated by your 1" rail "sensor."
One big problem though. Unlike photosensors, the prototype track circuit, and three-rail models, your rail scheme would only work with locomotives and/or lighted cars. This is the advantage of photo detection vs. current sensing. Whatever piece of rolling stock interrupts the photo beam will trigger it. Even cars with plastic wheelsets, or metal wheelsets with insulators, as needed for two-rail operation.

On a long ago HO layout I used a variation of your idea. No 1" isolated rail section, just a small springy wire over one of the rails. When wheels rolled over it it turned on crossing lights. Super-basic (I was still a kid) but it worked.

Regards;

Traction Fan 🙂


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