I would make it simple and cheap. A switch for the loop track changes the polarity.You can connect this switch with the turnout, if you want.

You have to stop at the turnout to let someone throw the turnout.

But I remember there're some electronic devices also.

Wolfgang

If I understand correctly, you want the reversing sections to change polarity automatically and the turnouts for the reversing sections to be thrown automatically too.

I think you would need to use a variety of train detection circuit for the turnouts to be automated.  Infra red, light sensing, and magnets all work to a degree.

Here is a link that might help.

http://home.cogeco.ca/~rpaisley4/LoopStall.html

This presumes that you are talking about a "teardrop" style loop with a single turnout controlling it.

Depending upon how you are controlling the turnout, it can be very easy.

If you operate the turnout with a Tortoise motor, the contacts on the Tortoise can be wired to reverse the polarity in the loop when the turnout is thrown. Since there will be a period when there is no power on the track as the Tortoise throws, the most realistic operation would be to wait at a signal (imaginary, if necessary) while the turnout throws. Since you are talking about minimizing cost, as you cannot afford to convert to DCC (PowerCabs are under $150) probably having two signals on each side of the loop controlled by the turnout would be pressing the budget, too!

If you are using a snap motor, you can use a switch with two more poles to reverse the polarity when you activate the turnout.

If you are using ground throws, you will need to make it a two step process. A DPDT switch to reverse the polarity in the loop and then throw the ground throw.

Hope this helps.

                              Bruce

 I have used DCC Specialities PSX-ARFB and was very happy with the operation.  Here is the link:      http://www.dccspecialties.com/products/powershield_x.htm

Cheers!

Ryan

Does this PSX_ARFB also handle crossings?

I have successfully used an autoreverser to handle frog polarity changes in a crossing.  The auto reverser I used was from Tony's Train Exchange about 8 or 9 years ago. 

In general an auto reverse unit is useful anyplace a train may encounter a voltage differential between the track that it is occupying and the track that it will be occupying next.  These place include

• reverse loops (from whence the name - auto reverser - comes)
• wye tail tracks
• crossings
• powered frogs in turnouts - certain autoreversers work for this, but they need to be able to change polarity when there is a short on only one rail. The Tony's reversers seem to work this way - other's may not. This is also a really expensive/complicated way of doing this! A simple micro-switch is much better.

Cheers,

Charlie