GWML ATP - Technical Details of the system

The GWML ATP uses small current transformers to replace the 2BA (or 0BA) links in the location cupboard in the aspect feed to the signal. That means that it's easy to add without any significant modifications to the existing signalling system.

I'll add more details tomorrow.

== Doublepost prevention - post automatically merged: 20 Jun 2026 ==

Overview of System



Signal Interface

There are two methods of interfacing with the signalling system, one for SSI systems and one for relay-based systems.

The SSI interface takes a direct 110V feed from the SSI signal module to the ATP enclosure.

The relay-based system uses a device called an ATP Light (or Lamp) Interface Transformer (ATPLIT) to interface the lineside equipment with the existing signalling system without infringing the integrity of the signalling system.

The ATPLIT (shown in the figure) is connected in series with the signal head transformers, across the last links before the signal head. It can be slipped like any other link. The ATPLIT is a current transformer and senses the current flowing in the signal lamp circuit.



The ATPLITs are normally installed in the existing signalling location case.

As it is a transformer, the ATPLIT provides electrical isolation between the ATP and signalling systems. It is fail-safe in that if a fault develops within it, the circuit across the link is maintained.

There are two sizes of link in use, OBA and 2BA. ATPLlTs are manufactured to fit both sizes.

Each size of ATPLIT is made in two types: one for use in the circuit of a signal aspect or position light, where the current is relatively low, and the other for use with a junction indicator, where the current is relatively high.

Photo showing some 2BA ATPLITs in service:


The use of ATPLITs enables an easy, quick installation to existing signalling systems while not affecting the safety of the existing signalling system.

No other changes are needed (other than providing a 110V AC power feed) unless the signal is a junction signal with a route or junction indicator. In which case, a new circuit is required to provide a positive proving circuit so that the ATP knows that the main route is set.

Is it more difficult to implement ATP on the actual trains?

Railperf said:

Is it more difficult to implement ATP on the actual trains?

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Yes. Fitting ATP requires more equipment, finding the power for that equipment. And more cabling. So a lot more work and hence more expensive (compared to fitting TPWS to a train).

That's one of the reasons that I think that all new trains from the 1990s onwards should have been specified to be ATP (or now, ETCS /ERTMS) ready (appropriate facilities provided so that at a later date, such equipment could be cheaply and easily installed).

And the (relative) cost is further reduced overall if new trains were built with the equipment already installed as and when they were ordered.

Annetts key said:

Yes. Fitting ATP requires more equipment, finding the power for that equipment. And more cabling. So a lot more work and hence more expensive (compared to fitting TPWS to a train).

That's one of the reasons that I think that all new trains from the 1990s onwards should have been specified to be ATP (or now, ETCS /ERTMS) ready (appropriate facilities provided so that at a later date, such equipment could be cheaply and easily installed).

And the (relative) cost is further reduced overall if new trains were built with the equipment already installed as and when they were ordered.

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My understanding is all new builds are now etcs compliant and have been since late 2010s

Useful and interesting. Thank you.

Bigfoot said:

My understanding is all new builds are now etcs compliant and have been since late 2010s

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It was introduced as part of future rolling stock procurement in May 2018:

Speaking an event in York on May 10 to launch the latest digital railway strategy, Grayling joined Network Rail Chief Executive Mark Carne in committing to ensure that all new trains and signalling procured from 2019 would be ETCS-equipped or at least ‘ETCS-ready’.

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[Article published 28th May 2018]

It doesn't seem to be a published requirement or standard but rather just a decision to include it as part of the tender specification. However, I don't know the full suite of standards so may have missed something!

zwk500 said:

It doesn't seem to be a published requirement or standard but rather just a decision to include it as part of the tender specification. However, I don't know the full suite of standards so may have missed something!

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Clause 5.3.2 of Rail Delivery Group ”Key Train Requirements - Version 5.1”:

For all other rolling stock, provision should be made for inexpensive retrofit of ERTMS / ETCS equipment (D).

In this context ‘inexpensive’ means that the design has made the provision for the fitment of equipment by the designer purposely allocating free space for internal equipment such as the Driver Machine Interface (DMI) and European Vital Computer (EVC); external equipment such as the Doppler Radar and Balise Reader; together with; power supply requirements and consideration of cabling to the relevant location(s).

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D365 said:

Clause 5.3.2 of Rail Delivery Group ”Key Train Requirements - Version 5.1”:

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Aha, thanks very much!

Annetts key said:

Yes. Fitting ATP requires more equipment, finding the power for that equipment. And more cabling. So a lot more work and hence more expensive (compared to fitting TPWS to a train).

That's one of the reasons that I think that all new trains from the 1990s onwards should have been specified to be ATP (or now, ETCS /ERTMS) ready (appropriate facilities provided so that at a later date, such equipment could be cheaply and easily installed).

And the (relative) cost is further reduced overall if new trains were built with the equipment already installed as and when they were ordered.

Click to expand...

An added complication is that the complex proprietary onboard equipment is no longer manufactured. The original supplier, Belgian conglomerate ACEC, now completely dissolved, sold its rail equipment division to Alsthom (still with an h) in 1989. The IET project is thought to have cleared out the remaining old stock in Alstom warehouses to fit the Hitachi trains.

It would be plausible to engineer an emulated implementation within ETCS computers and either have specific transmission modules interface with the original trackside transponders, or replace each of those with active ETCS balises and loops, but this would be a rather pointless and expensive exercise for an obsolete technology; the GWML being the only user worldwide today. Belgium had updated its own advanced warning and trainstop TBL1 systems to TBL1+, using ETCS transponders by 2012. That might have generated some used trackside spares for GWML. Similar equipment was in place on Hong Kong's East Rail Line until fairly recently, but that's it, GWML is now the sole installation.

MarkyT said:

Similar equipment was in place on Hong Kong's East Rail Line until fairly recently, but that's it, GWML is now the sole installation.

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Is it the case that TPWS only protects some signals? So outside of the GWML, the railways of the UK can be vulnerable to trains overrunning red signals without automatic braking taking place.

MarlowDonkey said:

Is it the case that TPWS only protects some signals?

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Yes, broadly speaking only those where there are points beyond the signal. Most automatic signals (i.e. generally where it's just plain-line without any pointwork) aren't fitted.

MarlowDonkey said:

So outside of the GWML, the railways of the UK can be vulnerable to trains overrunning red signals without automatic braking taking place.

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HS1, the Thameslink & Elizabeth line cores, and the Northern City and Cambrian lines all have ETCS or other equivalent in-cab signalling - but yes, most lines do not have the facility to prevent SPADs at automatic signals.

MarlowDonkey said:

Is it the case that TPWS only protects some signals? So outside of the GWML, the railways of the UK can be vulnerable to trains overrunning red signals without automatic braking taking place.

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Yes. Just over a quarter of all main running signals are equipped. They are almost all junction signals protecting head on, merging and crossing conflicts. Signals were equipped on a risk basis, using an established junction risk analysis model taking into account traffic density, passenger numbers, speed, length, type of rolling stock, etc. By concentrating on the highest risk locations with a very simple bolt on system, initial nationwide protection was provided very quickly, though it's fully acknowledged the system is not perfect. The logic was some protection sooner, then more modern ETCS systems will eventually provide full protection later as signalling is renewed.

And also note that not all trains that use the GWML are equipped with the ATP systems (e.g. Elizabeth Line), nor is every track on the GWML fitted, by quite some margin!

MarkyT said:

An added complication is that the complex proprietary onboard equipment is no longer manufactured.

Click to expand...

Yes, the ATP equipment used on the GWML ATP scheme is obsolete and I would not propose this equipment to be fitted anywhere else. It could be re-engineered, but that would cost money that could be better spent elsewhere and re-engineering it would take time.

At present, for new fitment across the network, the better choice would be ETCS/ERTMS which includes the safety features of ATP as standard. Network Rail has already started fitting this to some lines. The problem is that the roll-out is slow. A lot slower than was expected by many within the railway industry.

I started this topic to discuss the the technical details of the GWML ATP system and some of the ideas used that made installation ("retro fitting") to existing signalling systems easier than some would imagine.

I was not involved with ETCS/ERTMS other than being in a joint Network Rail and union working group for a short length of time.

I did attend the training course on the so called "passive" Eurobalises, as they are also used for other reasons. But I have never worked on or been trained on ETCS/ERTMS.

I have worked on and been trained on the GWML ATP system.

I do not and did not intend to have this topic as a discussion of which system is better. Simuarly I don't want this topic to be discussions about which is more cost effective or simular. There is already other topics where that discussion has taken place.

== Doublepost prevention - post automatically merged: 22 Jun 2026 ==

mr_moo said:

And also note that not all trains that use the GWML are equipped with the ATP systems (e.g. Elizabeth Line), nor is every track on the GWML fitted, by quite some margin!

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The lines that are fitted are the main lines between just east of Bristol Temple Meads (the first up line signal is the signal protecting North Somerset Junction) and London Paddington.

The South Wales main line between just east of Bristol Parkway station and the junction with the main line at Swindon.

Some passenger loops in the area described above are also fitted.

No, not all trains that use these lines are fitted. For GWR trains, it's only the 80X that are fitted.

To remove ATP* from the Chiltern network, TPWS train stop is fitted to every signal¹ and also include inner and outer over speed grids.

*Was removed due to the cost and challenges faced sourcing parts for both track and train.
¹Not on the LUL section. Only trip cock.

Annetts key said:

Yes, the ATP equipment used on the GWML ATP scheme is obsolete and I would not propose this equipment to be fitted anywhere else. It could be re-engineered, but that would cost money that could be better spent elsewhere and re-engineering it would take time.

At present, for new fitment across the network, the better choice would be ETCS/ERTMS which includes the safety features of ATP as standard. Network Rail has already started fitting this to some lines. The problem is that the roll-out is slow. A lot slower than was expected by many within the railway industry.
I started this topic to discuss the the technical details of the GWML ATP system and some of the ideas used that made installation ("retro fitting") to existing signalling systems easier than some would imagine.

Click to expand...

The pace of signalling renewals has slowed I think. The urgent safety jobs get done, but when a sizable chunk of south London gets resignalled, there's no incentive whatsoever to actively provide a final level 2 no signals solution as it provides no benefit for trains that still spend most of their time elsewhere in colour light areas and are mostly still unfitted. The realities of legacy fleet fitment have led to a tacit policy of allowing most old fleets to die out and be replaced by modern trains that are at least designed for ETCS, if not all fully equipped from new. Once all stock on a route is fitted, as will soon be the case on the ECML, then areas can start to be converted. As the fitted proportion of the broader national fleet steadily rises, it becomes ever more practical to do many more spot renewals anywhere using the technology, and also retrofit some 'ETCS-ready' areas more recently resignalled.

I recall NR HQ was firmly against any kind of active balise Level 1 solution in the early 2000s. They didn't want to interface to trackside signal circuits as they considered that complex and difficult, and also wanted to do away with the signals anyway. That's a fine long-term objective, but it means there's no easy off-the-shelf UK L1 solution that might be applied selectively where it makes sense, perhaps for an ATP replacement project, or rural lines where there are few signals anyway. I think wiring some connections from existing circuits in the location cabinet to an LEU is actually the simple part, and it must have been other factors that drove the perceived high cost of ATP vs TPWS, but the main variable was the number of signals equipped. TPWS demonstrated that if well organised, the industry could design and implement similar changes in thousands of equipment cabinets throughout the network in a short timescale, and other national railways used equivalent concepts to interface old signal circuits to LEUs that control trackside balises and loops.