Reed tracks

[Archie]

Could somebody be kind enough to explain 'reed tracks' many thanks.

[Peter]

Could somebody be kind enough to explain 'reed tracks' many thanks.

Reed systems are a form of FDM. The same transmit and receive principal is used for a reed track but instead of being sent down a cable, the given frequency is sent down the rails to be detected by the receiver. Discrimination and a certain level of immunity is achieved because the receiver is looking for the specific frequency.

Without looking it up, I cannot remember how many frequencies are used for these. They are available for use as jointed and jointless tracks. In the past they were used for dual electrified areas but there have been questions over their suitability for this. That said, all of the tracks for two miles out of Paddington (25kV OLE next to LUL DC traction) use them with no problem.

Hope that very quick overview helps, but if you have more specific questions, we'll see what we can do. They are covered in the Network Rail Signal Maintenance Specifications and the Train Detection Handbook.

Peter

[Archie]

Could somebody be kind enough to explain 'reed tracks' many thanks.

Reed systems are a form of FDM. The same transmit and receive principal is used for a reed track but instead of being sent down a cable, the given frequency is sent down the rails to be detected by the receiver. Discrimination and a certain level of immunity is achieved because the receiver is looking for the specific frequency.

Without looking it up, I cannot remember how many frequencies are used for these. They are available for use as jointed and jointless tracks. In the past they were used for dual electrified areas but there have been questions over their suitability for this. That said, all of the tracks for two miles out of Paddington (25kV OLE next to LUL DC traction) use them with no problem.

Hope that very quick overview helps, but if you have more specific questions, we'll see what we can do. They are covered in the Network Rail Signal Maintenance Specifications and the Train Detection Handbook.

Thanks Peter for your reply, I can source a copy of the SMS through work so I shall have a look.
Peter

[PJW]

Could somebody be kind enough to explain 'reed tracks' many thanks.

Reed systems are a form of FDM. The same transmit and receive principal is used for a reed track but instead of being sent down a cable, the given frequency is sent down the rails to be detected by the receiver. Discrimination and a certain level of immunity is achieved because the receiver is looking for the specific frequency.

Without looking it up, I cannot remember how many frequencies are used for these. They are available for use as jointed and jointless tracks. In the past they were used for dual electrified areas but there have been questions over their suitability for this. That said, all of the tracks for two miles out of Paddington (25kV OLE next to LUL DC traction) use them with no problem.

Hope that very quick overview helps, but if you have more specific questions, we'll see what we can do. They are covered in the Network Rail Signal Maintenance Specifications and the Train Detection Handbook.

Thanks Peter for your reply, I can source a copy of the SMS through work so I shall have a look.
Peter

The primary problem was with reed jointless; under some circumstances there was a wrongside failure mode. The track was fundamentally current operated and at the limits a piece of 37 core cable was placed as a loop between the rails. I think that one core was connected to the pair of rails effectively shorting them together and the remaining 36 cores were connected in series with each other so that the cable itself formed a 1:36 step-up transformer whose output was then taken to the location to drive the receiver.

Obviously a train on the track would need to shortout the signal- as I write this it does seems unlikely, unless there was also some series resistance. Perhaps it was the 36 cores that were connected as the primary of a step-down transformer.......

Anyway somehow it was possible for a train occupying the adjacent track to cause a track that should have been occupied by the train ahead to show clear; it occurred in particular for an auto section where trains likely to be on adjacent tracks and this was riskier as signaller had no control on the signals or even visibility of the aspects they displaying.

If I ever understod the mechanism then I have since forgotten, but I am sure that the incident was on the 3rd rail dc electrified Southern Region and therefore I think that it may also have been related to impedance bonds used to allow traction current to return to the substation but should have been blocking the reed TC signal.

The Paddington Reed TCs are jointed and a bit different to standard ones; I think that the Power Amplifier was different in some minor way perhaps by having another adjustment resistor; I do remember as a tester having a briefing on setting them up from Ian Harmon. Since this was nearly 20 years ago, perhaps I may be forgiven for forgetting the details.

The good thing about Reed TCs was that they WERE traction immune to ac and dc and at one time were about the ony TCs that could be used in such environment. Basically they were considered immune since receivers sensitive to a very narrow range of frequencies; the "Q factor" of the band pass filter is amazingly high and narrow- implemented mechanically by what are effectively a pair of tuning forksthat are meanically coupled. By chossing frequencies (in the approx 300Hz range) that not a harmonic of the 50Hz mains or its allowable frequency deviation, meant it could be regarded as ac immune.
However with ctrains now having variable frequency electronic drives, their interference spectrum can be virtually anything and certainly not linked to the a.c. mains frequency.
The presence of Reed TCs are always a problem when trying to get a Safety Case for a modern train to operate over a route having them. We don't install them new now and the HVI = High Voltage Impulse seemed like the solution for a while (until NR found out how much it cost to service the specuial relays they utilise) but they only operate over much shorter lengths.

Generally for dual immunity I have to say that axle counters have much to commend them.