Signal Controlled Speed Influence (HLU)

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Summary: Zimo's HLU, also known as Signal Controlled Speed Influence, allows control of trains based on signal states.


As this is primarily a European system, information in English is hard to come by. Most of the following is derived from instruction manuals and Zimo catalogs. If you can contribute additional information, please do so.

ZIMO digital systems offer a second level of communication for transmitting data to vehicles in specific track sections. The most common application is the “signal-controlled speed influence” for stopping trains and applying speed limits in 5 stages, with data sent to the track sections as needed in the form of HLU cut- outs prepared by MX9 track section modules or its successors. This feature only operates within ZIMO systems.

All Zimo decoders since 2004 support this protocol, as do some other European decoder manufacturers. Many DCC manufactures of European origin feature some form of automation.


Zimo HLU
Identifier Meaning
UH Interstage
U Ultra Slow
LU Interstage
L sLow
FL Interstage
F Full Speed

Zimo began developing the HLU system many years prior to the arrival of Automatic Brake Control (ABC), and in many aspects HLU is more capable than ABC.

Its name is derived from the H-L-U signals transmitted by Zimo's accessory decoders.

HLU is a second communication protocol, from an electronic unit, located between the command station and an insulated track section, to the stationary decoder located on the track section. Zimo calls theirs a StEin.

HLU data can be different from track section to track section (e.g., concerning HLU-limits), does NOT have an address and are read by ZIMO decoders.

HLU data works like commands to stop trains or to reduce their speed to one of the 5 HLU speed limits. HLU data reaches the decoder virtually without delay, because it is sent about 11 times/sec.

One of the 7 HLU steps is set on the stationary decoder's track section outputs, usually based on the command of an interlocking program (i.e. computer). When the train passes a point contact, the StEin switches the HLU-steps autonomously.

The speed limits “U” (Ultra low) and “L” (Low speed) as well as the intermediate limits of the “signal-controlled speed influence” can be defined

Six speed limits (including stop) slow down every train equipped with the appropriate decoder to the (adjustable) maximum speed of the track section, or stop it.