I tried a Gaugemaster automatic reverse loop module but no matter what I tried - shortening the leads, shortening the section of track, I couldn't to get it to work 100% reliably.  It would work if the loco was travelling in one direction, but then it would fail in the other.  Sometimes it would cause the loco to stall momentarily, and then accelerate away from zero.  Other times it would require a reset of the controller. Maybe it was the combination of the Hornby Elite controller and the Gaugemaster reverse loop module.  I don't know.  Anyway, I gave up trying.  I'll try to use it again on the electrofrog diamond crossing that I have.

On the diagonal reverse loop, I planned to have a station, and I decided that no matter what, locos would have to stop there.  The point arrangement is complex enough for me to welcome a brief pause in the proceedings in order to work out what to do next anyway !

Sour Grapes Station, I think I will call it.

But a single switch to reverse polarity wouldn't provide the safety that I wanted.  In effect, I wanted a dead section of track between two sections which had different polarities - so that an accidental short did not stop everything from working.

My reverse loop has, in both directions, the following sequence - point, track section, track section / station, track section, point a break in electrical continuity between each section..  This was done out of necessity as I have a diagonal track crossing a baseboard join.  In order to manage this, I have a small rectangular board across the join which lifts out, thus enabling a 90 degree cut to be made across the track itself.  This is shown on the section of track plan, left.

The switch is a miniature 4 pole double throw device with centre off.  ie four separate switches which are all put into the same position at the same time.  I have labelled these, 1,2,3 and 4.  Each 'pole' or switch has 3 connectors, which I have labelled on the left as A, B and C.  So when the switch is in the 'up' position (see note below) - B1 is connected to A1; B2 is connected to A2;  B3 is connected to A3 and B4 is connected to A4.   When the switch is in the down position, B1, B2, B3, B4 are connected to C1, C2, C3, C4 respectively.

Operation

A train enters the station from the bottom of the diagram.  The switch is in the down position.  It is running perfectly well on the main line with the black and red power feeds from the controller - which come from the same place as the black and red feeds at the top of the diagram.

So the loco enters section R3 from the main track with the black (-ve) lead on its left and red (+ve) lead on its right.  The switch is down so Row B is connected to Row C.  The left track (grey lead) is connected to the black lead in switch B4.  The right track (blue) is connected the red (+ve) lead at B3.
Similarly, R2 is connected with the same polarity - Grey to Yellow through B1/C1 and Blue to Brown at B2/C2.  So both R2 and R3 have the same polarity as the main track to the south.  The train is stopped at the station.  If it fails to stop, then it will enter R1.  No short will take place, as the switch is down and R1 is not connected to anything on Row A of the switch.  All four switches are connecting Row B to Row C.  The loco will continue along R1 until all of its pickups have left section R2, and will then stop dead.  Each section is long enough to hold a loco.  Only the loco at the front has pickups on my DCC layout, otherwise I would need very long track sections.

When the train has stopped, the switch can be thrown so that A1, A2, A3, A4 are connected to B1, B2, B3, B4 respectively.  Section R3 becomes dead and sections R1 and R2 are both connected to the section to the north of R1 - ie with a reversed polarity.

Simples !

(nb - in reality, when contacts on row C are connected to their respective centre contacts, the switch lever is in the 'up' position.  It is easy to make this error, but the solution is simple - rotate the switch through 180 degrees !)

Footnote.  When operating in DC, it necessary to stop the loco, throw the 4PDT switch AND change the direction of the controller before entering the last section.  Using DCC, there is no need to change the controller - in fact the 4PDT switch can be thrown while the loco is moving in the middle section.