Automatic Brake Control

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Automatic Brake Control (ABC) is a system to automatically apply the brakes to a locomotive when it enters a special section of track. This could be at a signal, at a terminus, at a junction or anywhere you want a train to stop automatically. The goal of Automatic Brake Control is similar to the Automatic Train Control (ATC) systems used on real railways to prevent trains passing a signal at danger.

Automatic Brake Control was developed by Lenz but has been adopted by other manufacturers such as Zimo. It is not part of any DCC standard, but provisions have been made to accommodate the feature in the DCC standard as an optional feature. Automatic Brake Control is fully interoperable

Automatic Brake Control-enabled locomotives will operate normally on a standard DCC layout, and standard DCC locomotives will operate normally - although obviously they won't stop automatically.

Only mobile decoders which are Automatic Brake Control-ready and have the function enabled via the appropriate CVs will respond to to automatic brake control.

Decoder Automatic Stopping Configuration Variable

Some DCC decoders support the optional Decoder Automatic Stopping Configuration, controlled by CV 27.

CV 27 can be configured to tell the decoder what conditions are needed to activate this feature.


ENABLED means the bit is set to a value of 1 (or ON). DISABLED means Zero.

Bit 0: Enable/Disable the Auto Stop when an Asymmetric DCC signal is encountered. If Enabled, the auto stop is trigged by a asymmetrical DCC signal which is more positive on the right rail.

Bit 1: Enable/Disable the Auto Stop when an Asymmetric DCC signal is encountered. If Enabled, the auto stop is trigged by a asymmetrical DCC signal which is more positive on the left rail.

Bit 2: Auto Stop in the presence of an Signal Controlled Influence cut-out signal.

Bit 3: Unused at the time.

Bit 4: Auto stop when reverse polarity direct current is present

Bit 5: Auto stop when forward polarity direct current is present

Bits 6 and 7 are reserved for future use.

If a decoder does not support one of the above features, it cannot allow the bit to be set improperly, meaning that bit will remain at its default value.

How It Works

Automatic Brake Control uses Asymmetric DCC to signal to the locomotive's decoder that it has entered a special section of track. This will bring it to a halt using a pre-programmed braking distance. If the asymmetric DCC signal is returned to normal, the locomotive will resume its previous speed.

The ABC section of track must be long enough that the locomotive plus it's stopping distance fits entirely within it.

Automatic Stop Signals

An Automatic Brake Control section can be placed infront of a signal so that a train automatically comes to a stop infront of a red signal. When the signal turns green, a switch closes and bypasses the asymmetric DCC module. The train will then automatically resume its previous speed using a pre-programmed acceleration curve. The switch can either be one half of a DPDT switch used to control the signal itself, or a relay controlled by the same circuit that controls the signal.

The ABC section can also be placed after the signal if you prefer to drive and brake the train yourself. Then it becomes a protection system and only stops the train if you attempt to pass a red signal.


In the diagram above a DPDT switch is used to operate the red and green signal lamps, the other half of the switch is used to short out the asymmetric DCC generator when the signal is green. The upper part of the diagram shows the switch in the stop position with the asymmetric DCC active, the lower half shows the same circuit with the switch in the clear position.

If the signal is a semaphore arm operated by a manual lever, a turnout motor (such as a Tortoise motor) or a servo, then the switch can be an SPST switch mechanically connected to the lever, the auxillary contacts of the turnout motor or a microswitch operated by the servo. In all cases, the switch contacts must be open when the signal is at danger.

[need some pictures here]

Obviously, you could also simply use a completely independent switch to control the ABC section but that would defeat the purpose of it being an automatic system. You would have to remember to throw the switch whenever you change the signal.

Directional Control

Asymmetric DCC allows directional signalling because the decoder is able to differentiate between left and right (or positive and negative) asymmetry. With the locomotive facing in one direction, it will see asymmetry in one direction, with it facing the other direction it will see the opposite asymmetery. And this means physically picking up the locomotive and turning it around to face the other way (or turning around on the layout if you have a reversing loop or turntable), it does not mean just reversing the throttle direction.

An Automatic Brake Control-enabled mobile decoder can be setup to respond differently depending on the sense of the asymmetry that it sees.

Essentially this means "is the asymmetry on the left rail or the right rail?" Directional control allows a train to ignore a signal facing in the wrong direction. The Lenz convention is to always use the right rail to signal Automatic Brake Control.

Terminus Stations

A terminus station can be fitted with a permanent Automatic Brake Control section so that a train automatically comes to a halt when it enters the staion. No other special controls or overrides are needed because the default behaviour of an Automatic Brake Control-enabled decoder is to allow reversing back out of an ABC section. So to leave the station, all you have to do is reverse the direction of the throttle and the train will start back up and resume its original speed.

Junction Protection

Automatic braking can be used to protect junctions where the turnout is set against a train. This can help avoid collisions and prevent derailment. Automation can be achieved in a similar manner to signal stops but instead the turnout control is used to switch off the asymmetric DCC.

Tortoise switch machines have two auxiliary sets of contacts; one set is commonly used to switch the frog polarity and the other is used for indicator LEDs on a panel or for signal LEDs. The second set can be repurposed for ABC control if the LEDs are wired in series with the Tortoise motor instead of using the contacts. Refer to the Tortoise user manual for more information.

Both legs of a junction can be protected with a single set of Tortoise contacts provided that you gap the same rail on both legs. In the wiring diagram below, a Tortoise machine is used to mechanically operate the turnout and uses one set of contacts to switch the frog polarity and the other automatically controls the braking zones.


Slow Approach

The Lenz BM2 also supports Slow Approach mode for use when a signal is at caution (yellow) rather than stop (red). This slows the train down or allows the train to continue at slow speed but does not stop it.

Lenz have not disclosed how slow approach works and it is not supported by other manufacturers' mobile decoders or asymmetric DCC modules.

Special Considerations

Metal wheels on rolling stock can potentially bridge the gap at the start of the ABC section which will effectively bypass the asymmetric DCC circuit and cause the locomotive to resume its original speed.

This can be cured in one of two ways; either extend the ABC section so that it covers the entire length of the longest train expected (plus stopping distance) or fill the rail gap with something like epoxy resin so that it is level with the top of the rail. This will prevent a metal wheel falling into the gap and making electrical contact with both sides. If you have rolling stock with multiple pickups for lighting then only the first solution will work.

Comparison to Simple Rail Isolation

A crude form of Automatic Brake Control can be achieved with just an isolating switch to cut power to one rail. When a locomotive enters the special section of track it simply loses power and immediately comes to a halt.

While simpler, it has many disadvantages compared to proper ABC.

  • Attempts to pass a signal at danger will result in a very unrealistic sudden stop.
  • Attempts to stop smoothly at a danger signal may cause an unrealistic jerk if the isolated rail is reached.
  • It will not be possible to back up from a signal at danger (for shunting movements).
  • All other features (sound, lights, etc.) will immediately stop working
  • A locomotive equipped with a high capacity Energy Storage module might be able to make it through an isolated section and then resume its previous speed.
  • Any type of bidirectional communication will be impossible.
  • The train will "disappear" from any occupancy detection systems that rely on current draw.

True ABC, solves all of the problems listed above

Available Automatic Brake Control Products

Mobile Decoders with ABC Support

Tam Valley Depot

Tam Valley Depot makes a number of interesting DCC products, including a DCC Train Shuttle. It can be used for end detection, a station stop, or to send accessory commands.

Automatic Brake Control Asymmetric DCC Generators

  • DIY
  • Lenz BM1 - a vey basic asymmetric DCC generator
  • Lenz BM2 - block stopping module with asymmetric DCC, slow approach, signal sensing inputs and occupancy detection
  • Lenz BM3 - block and signal module with asymmetric DCC, signal driver outputs and occupancy detection

See Also

External Links

Tam Valley Depot