Is there some signal or voltage polarity that indicates this direction?

I use a simple DC control that is derived from full wave rectified AC and there is a switch in the handheld controller that I use to reverse the polarity and direction. I want to monitor the track to automatically enable signals in that direction. So in answer to your question, there is only the polarity of the track supply as a source.

 

OK, now that there is some more information provided, one more question: Is there a separate power source, isolated from the tracks power , that is available?? such as might supply lighting for the buildings, or power to drive the switches and decouplers?? It seems that the addition needs to sense both the presence of the train and it's direction of travel. Direction is easily found by the polarity, presence of the train is less easy. Is the presence of the train sensed elsewhere in the layout?? If so, by what method??
Thus far I have seen no response to any of my suggestions for optical sensing, and no hint that any method of sensing the train presence has been mentioned.

I will try to answer some of your questions.
The layout is all analogue. I have multiple supply sources available for signalling and point control.
The signals I have made respond to the train's passage, not the other way round.
The controllers simply amplify a full wave rectified DC supply. This is the supply I wish to monitor.
I want to monitor a loop of bi-directional track that I run trains on in both directions such that when I start the train in one direction then the signals that show train passage in that direction are energized.
Similarly, when I reverse the train's direction then the other signals for that direction will be enabled.

 

I use a simple DC control that is derived from full wave rectified AC and there is a switch in the handheld controller that I use to reverse the polarity and direction. I want to monitor the track to automatically enable signals in that direction. So in answer to your question, there is only the polarity of the track supply as a source.

To clarify. I have other supplies that I use for signals and point control.
The track supply, at the track, is the source I want to monitor for polarity.

 

A suggestion...
Connect two optocouplers, with the opto input diodes connected in an anti-parallel configuration to the voltage feed to the track. Then connect each opto output to a relay. Each relay will then energize based on the track voltage polarity. The optos will use very little current from the track supply (~5-10mA).

Or, if more current is available, use two relays, each connected via a serial diode so as to detect polarity, connected to the track supply. Each relay will then energize based on the track voltage polarity.

Then use contacts of the relays to light the appropriate signals.

 

I would like to monitor the track supply as it varies between zero and 12 volts and switch a relay whilst one polarity is present to enable signals for that direction only.
I would like it to detect voltages over 1.5 volts up to a maximum of 12 volts. This being the speed control window.

Optocouplers T1 and T2 provide isolation from the track voltage and polarity.
IC1 activates relay RL1 and latches when in Forward Polarity at appx 1.5 volts with the components listed.
Same for IC2 when in Reverse Polarity.
If RL1 or RL2 is activated it resets the other.

 

Optocouplers T1 and T2 provide isolation from the track voltage and polarity.
IC1 activates relay RL1 and latches when in Forward Polarity at appx 1.5 volts with the components listed.
Same for IC2 when in Reverse Polarity.
If RL1 or RL2 is activated it resets the other.

View attachment 349618

Thank you so much for this circuit.
Questions: I only need one set of changeover contacts to supply appropriate signals. Is it possible to have this circuit continue to operate if the second relay RL2 is simply omitted? Is it possible to use the output from the second optocoupler to reset the first 555 without the need for the second 555?
I only ask because I need to make 5 sets of these polarity monitors for 5 separate sections of track that can be supplied from different controllers.

 

There is a way using the other opto to reset.
Making dinner right now, will post soon.

 

There is a way using the other opto to reset.

 

View attachment 349627

Thank you. This will be much easier to make. It is very useful that you have included the component part numbers for purchase.

 

Those components in the schematic were what I used to bench test the circuit.
Here is a link to an inexpensive 12 volt relay that can be activated directly by a NE555.
Relay