As more and more railway signalling is moving to either ETCS or CBTC, some of the legacy (but very interesting!) signalling schemes might be forgotten.
This page is a short summary of some of them, along with links to archived documents that might otherwise be very difficult to find.
The following terms will be used quite a lot on the page:
The fundamental principle of the VAL's ATO and ATP systems is based on observing phase shifts caused by cables which are crossed at specific intervals (with this interval being varied according to the target speed for a given section).
Various subcarriers are also transmitted on these cables, for other ATP / ATO / ATS
The best resource for the VAL's signalling system is a report published by the US Department Of Transport, with the title "Interim Assessment of the VAL Automated Guideway Transit System"
An Archived copy of this document can be found here
There's also a reasonable summary of the system in the publication "VAL Automated Guided Transit Characteristics and Evolutions", by Regis Lardennois (some of the frequencies and bitrates in this document conflict very slightly with the numbers in the US DOT report).
Where there is a conflict, the number from the US DOT report is shown first, and the later publication shown in brackets.
| Name | Frequency | Modulation | Notes |
|---|---|---|---|
| Safety Frequency | 42kHz | Phase Modulated with: One of the "Group 1" frequencies, and One of the "Group 2" frequencies |
This frequency is always present during normal operation. Removal of this frequency results in emergency braking. |
| Stopping Speed Program | 33kHz | None | May or may not be present. |
| Station Data Link | 36kHz | FSK | Present at stations only. Placed on the side of the track adjacent to the platform doors. |
| Voice + Telemetry | 135kHz | Phase Modulation + FSK subcarrier at 5.5kHz | 4kHz voice bandwidth (-6dB) 192 (200) bit/s biphase data |
Group 1 frequencies specify the direction of the train using either a 440.5Hz tone to indicate "Direction 1" or a 532Hz tone to indicate "Direction 2" (absence of phase modulated subcarrier will result in emergency braking).
Group 2 frequencies specify the operational mode:
Loop crossings (causing a phase shift) are spaced such that if a train is travelling at the correct speed, it will see a loop crossing every 0.3 seconds.
Loop crossings at an interval slower than this cause the train's target speed to increase, and loop crossings faster than this decrease the train's target speed.
If the interval between loop crossings is 0.27 seconds or less, emergency braking is triggered.
An illustration of this concept can be found on page 107 of the US DoT report.
| Name | Frequency | Modulation | Notes |
|---|---|---|---|
| Presence | 69kHz (69.4 - 70kHz) | None | Emergency alarms activated on the train will remove this carrier |
| Station Data | 55.25kHz | FSK | BiPhase at 128bit/s |
| Telemetry | 80kHz | OOK | Return to Zero, 384bit/s |
| Voice | 100kHz | Phase Modulation | 3kHz -6dB cutoff |
When it opened in 1987, the DLR used a fixed-block signalling system (that has since been replaced by SelTrac).
Very little information about this system (apart from it being a fixed-block system, manufactured by GEC-Alsthom) exists on the internet, but there is one archived document here
This system has a few points in common with the VAL's signalling:
However, the ATO aspect was very different - it used a table of "route speed profiles", which specified distance/target speed pairs to be used.