To clarify

That is correct

That is correct. Seems to be an easy method for basic station stopping automation. Have seen the videos that show how to implement with a relay and signaling to bypass the diodes on a green signal.

Welcome to my Pain...

Dear Dan, Joe,

Joe, Yes, "ABC" aka "Asymetrical DCC Braking", as described per the NMRA DCC Spec for CV27
https://www.nmra.org/sites/default/files/standards/sandrp/pdf/s-9.2.2_decoder_cvs_2012.07.pdf
/> (Section 9.2.2, Page 5)

Dan, welcome to my angst... For "reasons" seemingly tied up in:
- "The Lone Hero Engineer" mindset (both modeller and romanticised railfan)
- "Who cares about signals/automation?" mindset
- "I don't want anything running my (model) train but Me"

the US market, and the homegrown decoders manufactured to serve it, appear to give CV27 and it's NMRA-specified features a short "don't care, not relevant" shove, and move right along... 

ST and TCS decoders can be configured to "stop on reverse-polarity Analog",
but given the dearth of basic (decoder and layout) wiring discipline,
and market-understanding of what wire goes where, and why, 
the likelyhood that any given decoder would be wired up with the required accuracy to allow said systems to work predictably/reliably, is, shall we say, dubious...

(HINT: If you think using the "Reverse direction" bit in CV29 is the Correct Fix for swapping mis-wiring the Orange/Grey Motor feed wires, as opposed to actually wiring the loco correctly, then such "decoder-based braking" systems are not for you...)

Bonus points, a quick read of the TCS Comprehensive Programming Manual
https://tcsdcc.com/file/5973
/> also suggests that enabling "Brake on Analog DC" (CV182, page 3) disables "Keep Alive" operation,
which becomes a real "sticky issue" for many modellers...

ESU decoders do support both "Brake on reverse polarity Analog" and "Asymetrical DCC",
(without disabling or compromising the operation of KA/CK/SA/Power-Packs!!!)

which is no surprise coming from a Euro background and primary market. Thank goodness ESU provide US-outline Lighting and Sound options, which bring "Euro featureset" options to the US market...

...However, given that Matt H (ESU USA) still has not made-good on his tease (circa 2019) that "LokSound V5 could gain Audio EQ onboard",

https://model-railroad-hobbyist.com/node/37113

I am personally regularly torn between:
- needing audio EQ to achieve properly scale-sound results, and work with the widest-variety/optimisation of speaker and enclosure installs
- needing Asymetrical DCC Braking for my regular exhibition layout builds, to automatically and deterministically handle things like trains stopping on dead-end drop-leaf staging, hung 4' above an exhibition-hall floor...
(and, that frankly, analog DC was doing handily for many decades before DCC was even "a thing"...)

https://model-railroad-hobbyist.com/node/5122

https://model-railroad-hobbyist.com/node/6281

Now, maybe the moment I mention "Signals", "Automation", or "Exhibition Layouts" as common reasons for ABC braking, it becomes all-too-easy for the Rank-n-File to respond:

"...ah, those are obvious Prof-specific edge cases that don't have any Real-World relationship to US model RRing experience..."

but the self-same Asymetrical DCC Braking system is an almost tailor-made solution to the myriad:

- "I have a drop-leaf accross a doorway, and fear the trains won't stop when it's in the Open Position"

- "I rely on Keep-Alive/Stay-Alive/Current-Keep units in each loco, and fear an uncontrolled run-away"

- "I have hidden-staging, and want to automate the movement/stopping of otherwise-unmanned trains"

- "I realise that simply "dropping track power" is a poor way to stop a DCC-controlled train, esp when KA-equipped, but don't know of any better solution"

- "I need to brake either single-locos or MU-consists, easily, automatically, and without having to customise the detection/triggering/braking systems to recognize specific locos..."

- "I want a system that works for any loco, oriented in any track-direction, without having to specifically program the Braking System to 'recognize that specific loco',
and without having to reprogram the loco with some 'address-range/duplication/consist-mask fudge'..."

questions which are commonly posed here on MRH and elsewhere...
(IE Asymetrical Braking has Practical Utility well-beyond "exhibition layout" edge use-case,
European Model Railway headspace, or "those crazy people who prefer fully-signalled/semi-automated layouts" deployments).

Personally, if ST added ABC (fully implemented CV27, per NMRA spec) to the TSU family of decoders,
then I would have near-enough to my personal "perfect" decoder...
(and my current roster of ESU LokSounds would start feeling justifiably nervous!)

...but it's ST's decision who they engineer their offerings for,
and it's my unfortunate duty to have to buy something-else in consequence...
(define the problem/use-case,
determine the capabilities of the potential options,
choose and deploy the Most-Appropriate Solution...)

Dan, not sure if this is what you were asking for, but it's an issue I've put a lot of time and effort to contemplating, wrangling, and making-my-piece with... (NOT a typo!)

Happy Modelling,
Aiming to find the perfect US-outline decoder, with Onboard Audio EQ and Asym braking, One Day...
Prof Klyzlr

PS have discussed the utility of ABC before, but my current Exhibition Layout build actively relies on Asymetrical DCC Braking, and therefore functions as a complete "worked example". Combined with an NCE Powercab and MiniPanel, I have something which is vaguely the equivalent of the old (Bulletproof!) Analog "Dumb Timer" semi-auto system for exhibition layouts. Single-button toggle between "Full Manual drive" and "Auto-shuttle", full DCC "triggerable effects" control maintained in both modes, and a helluva-lot-less "constant nervous angst" and "mindless repeditive hitting of the Fwd/Rev buttons" for the Show Ops Crew on a P2P DCC layout...

Braking

Probably because decoder designers and manufacturers don't want the additional complications (and expense) of designing a decoder which can detect a symmetrical or asymmetrical DCC signal. Lenz also has their Automatic Brake Control, a more sophisticated evolution of their patented asymmetric DCC. Not sure if Lenz demands a royalty.

There is also the braking district, using a special booster that only transmits a "stop" when active, which can also be done using some Digitrax command station program outputs too. 

Some advocate for disconnecting power to the approaches to a lift out. Works great, until a unit with a keep-alive on board comes along...

if they can "brake on Analog"...???

 Dear Michael,

...which would hold water, if the same decoders weren't (already) able to detect and "brake on" Analog...
(IE "detection of what kind of signal/voltage is at the rails, and polarity of same" is apparently already implemented, and it only takes < 1.2VDC offset to provide/present a clear "apply brake" condition to the decoder).

...which is exactly a use case which Asymmetrical DCC Braking addresses...

- Loco Automatically Brakes when approaching "the gap" 
(and the diode-pack is cheaply, easily, automatically switched In/out of circuit by a simple microswitch,
physically triggered by the lifting/swinging of the "bridge"...
...no additional/manual intervention required...   )

- Full normal User control is maintained to "back away from danger"
(Can still blow horn, change lights, just cannot proceed toward Danger)

- ...irrespective of the actual orientation of the loco on the track
(Does not matter if the loco approaches "Danger" in Fwd or Rev direction, the braking automatically works)

- and the "braking distance" can be controlled/override CV4 "decel" values,
avoiding "it's Braking, but won't stop in time" fears...

Indeed, given the pervasive advice

"Always Disable 'Run on Analog', as it is the Most-common root-cause of uncontrolled/unexplained Runaway of DCC locos",

I would have thought that implementing "Brake on Asymmetrical DCC" would have been preferrable over "Brake on Analog"...
 

Happy Modelling,
Aim to Improve,
Prof Klyzlr

Another point that puts US behind European manufacturers

This just shows how we fall behind European modelers. We have some fantastic products but after visiting a modeler that models a German HO railroad, I have seen how far behind we are here.

Not only Asymmetrical DCC braking but locos that have diesel smoke generators, locos with operating cooling fans, the interesting automatic automobile street system. All NOT available from American manufacturers.

JMRI and particularly those who now do remote operations over the internet are very good systems. The problem is that many do not need or want to get into the complexity of programming while still wanting to have some basic automation. Whether to protect a lift out, show an exhibition layout , or just run multiple trains by themselves, the manufactures can ,but choose not to, support these things.

Time to write some emails and do some talking at the next national train show.

Prof is right

...and yet...

Dear Joe,

...and yet, in a classic example of "cut off nose to spite face",
these pervasive "we don't want no stinkin' automation" feelings mean many modellers typically (have to) resort to very over-complex solutions to drop-leaves, swing-doors, hidden-staging, and other model-thought layout-design/op issues which "break the immersion" and "kill the illusion" of emulating the prototype,
because the modeller is obliged to "think model-RR thoughts" when navigating beyond the "scale model scene" environs...

Pls don't mishear me, I'm not personally keen on stuffing a model RR with "tech for tech's sake",
and I certainly don't want to take the throttle out of the hands of the modeller who enjoys playing "the role of Engineer", 

but tasteful/appropriate tech deployments that can "do the bits the Human doesn't want to be distracted by" or "do the 'behind the scenes' parts" so there's always a train on the Departure-track, ready to roll accross the Sub,
(IE work alongside the modeller, to support the action,
think R2-D2 taking care of the X-wing's 'operational needs' while Luke focusses on the flying),

that's gotta be a fair-game deployment-opportunity for ABC, and similar...

Happy Modelling,
Aim to Improve,
Prof Klyzlr

Automation

Having a signaled, semi-auto, toy train operation, I can attest to the benefits of ABC. Not only does it allow braking, trains can also have 'slow approach' speed limits. Multiple trains can operate in the background without colliding while you can have full control of another train for switching or what have you. The European sound decoders ( ESU and Zimo ) can have any number of sound projects loaded, although perhaps not varied enough for US audience (?).  

Automation use

It’s just not very common in the US.  I don’t think the situations prof K outlines where it would be really useful are all that common either - there are certainly gates across doors and such, but not nearly as often as it might seem from reading forums.  I’ve operated on a number of layouts in the area of all sizes, and I can’t of one where there was a gate or something that needed to be opened during operation.

Staging automation is an interesting concept, but I suspect one that works out better with the rest of the railroad automated too, the layouts I’ve operated on needed judgement on what to stick where in staging given how things were actually working out during the session, as opposed to what the plan was.

I certainly wouldn’t say no to a decoder that had automation capability, I’d enjoy messing with it, but it’s never going to be the governing factor in choosing a decoder for me.  It is unfortunate that all decoders don’t have all features, but I completely understand why US decoder manufacturers haven’t felt much pressure to add automation features yet.

Sidebar - liftouts, gates, and drop sections

This doesn't have anything to do with the ABC/Asymmetrical DCC braking part, but anyone who puts a removable section of track in their layout and does not include some sort of physical track block.... is letting Murphy run their railroad. 

It doesn't even have to be (although it would ideally be) automatic. It could be as simple as a couple of pins inserted at the entrance to the Grand Canyon. 

"It should have... ", "It wasn't supposed to...", and "It was designed to..." can be three of the most frustrating phrases in the English language.