Idea for protecting a switch thrown against incoming train

[nickelplate759]

I had an idea (uh-oh) on a way to keep a train approaching a switch thrown against it from getting too close.
I know there should be signals, etc., and alert operators, but just in case....
I also know that there should be short protection - this isn't meant as a substitute for that.

The idea is to prevent a train (or at least an engine) from getting too close to a switch thrown against it.
Using a DPDT (like what is built into a Tortoise), power is removed from the rail that doesn't match the polarity of the frog - this requires that the frog polarity be controlled  via the other side of the DPDT.
Here's a crude diagram:
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Rail sections A and B are only powered when the Frog polarity matches the polarity of the rail to their left.
If A and B are longer than a locomotive, then I think this could work.

I want to do this to protect some switches that are hidden from the operators view.

I can also see some problems however.  First, if there's a keep-alive in the engine, it will just sail on through.  Second, it doesn't protect  against backing a train into the open switch.  So since it isn't foolproof, maybe it's not a good idea.      I'm posting it soliciting constructive criticism.  Fire away!

[Point353]

How far will the keep-alive capacitor allow your locos to coast?
Is it possible to make the 'A' and 'B' sections long enough so that a loco will eventually stop within their length?
You'd need to make the  'A' and 'B' sections a full train-length long to prevent back-up moves through a misaligned turnout.
Maybe you need a camera and monitor to provide a view of the hidden section.

[jagged ben]

The diagram is ... crude ... but I think I get it and I like the idea.

To clarify, you'd wire the frog as normal.  Then you'd take the other switch, feed the same power as the frog to it, and let it switch on A or B depending on position.  Correct?

[nickelplate759]

Correct.  And I freely admit the drawing is crude. I'm no draftsman, nor a circuit designer.

[Maletrain]

I like the idea.

I have used a similar concept with a reversing section of a wye, where I could not provide enough length to make sure that the section is longer than the longest train.  So, I make a dead section on the side of the wye that is not polarity-matched to the reversing section.  I use the same switch to reverse power and to select the dead and powered sections, so I need a QPDT with two sets of poles cross connected in the usual "X" fashion.

In my case, ths has caught some operating goofs and prevented the short circuits that otherwise would have occurred.  But, it won't work with large capacitors on the loco motors.

[boisecity]

Yes, that works very well.  For N scale there wouldn't be many locos with stay alives.

You would need to educate your operators so that if their train suddenly stops short of a turnout they don't wonder why it stopped.

JohnF