Yellow Banner Signals
The home signals at Minffordd and Tan y Bwlch each have an unusual yellow banner signal, in addition to the red and green colour-light aspects. The home signals at Minffordd also have yellow colour-light aspects, that have not been commissioned. This explanation is derived from an article by Peter Matthews in IRSE News.
Automatic Signalling[edit source]
The signalling at both Minffordd and Tan y Bwlch loops is automatic, the relay interlockings being operated by the track-circuits and the staff instruments. The automatic signalling only ever routes trains in "right road" (which on the FR is right-road, as right-hand running has been observed at both Minffordd and Tan y Bwlch loops right from the earliest gravity-train days).
A train approaching the station, on occupying the approach track-circuit, sets the route for itself to enter the loop. For a train to leave the station, taking a staff out of the staff instrument will motor the points and clear the starting signal. Once the departing train has left the station, then the points will self-restore back to normal, ready for the next arrival.
When one train is going through, it will have to stop so that the loco-crew can change tokens. When two trains are crossing, the crew of the first train to arrive will put the token for the section they have just left into the instrument, then immediately take it back out again ready to give to the second train coming the other way: this sets the route right through the station ready for the second train. The second train could then in theory pass straight through without stopping (if not timetabled to stop).
Signal Aspects[edit source]
The signalling at the automatic loops is 2 aspect, both the home and starting signals only show red or green.
The first station to be automated was Tan y Bwlch, in 1987. Despite the fact that almost-all trains stop there for passengers, have to do a token exchange, and the speed limit through the loops is only 10 mph, the HMRI Inspecting Officer required that trains either had to be cautioned at the home signal if the platform starting signal ahead was at danger, or an indication given of whether the platform starter was on or off. The FR didn't want to have to caution trains at the home signals, as this would have meant unnecessarily delaying just about every train, so having a few spare banner signals available decided to use these to repeat the indication of the platform starters. These yellow banners are officially termed "Caution Signals". A yellow-banner caution signal will only clear to its proceed (off) aspect if its home signal and the platform starting signal ahead are both off (both showing a green aspect).
So why not use a yellow colour-light aspect to show if the signal ahead is at red? Well, on the FR, yellow and green aspects don't indicate what the signal ahead is showing: on the FR, yellow means "proceed at slow speed", while green means "proceed at normal speed". This meaning of the aspects was used at both Tan y Bwlch and Minffordd before automation, and it is still used today at Rhiw Goch, where the home signals show green for the main-line, or yellow for the loop - regardless of what aspect the signal ahead is showing. It is quite usual for a home signal to show green with the next signal ahead showing red. Tan y Bwlch couldn't be commissioned with yellow and green aspects having different meanings to those at the other passing-loops on the line, hence the use of the banner caution signals.
Minffordd 3-Aspect Signals[edit source]
When Minffordd was subsequently automated a year later, 3 aspect heads were fitted to the home signals, rather than 2 aspect heads as provided at Tan y Bwlch. However, only the red and green aspects are used, not the yellow. The idea was that when Rhiw Goch was abolished, as looked likely at that time, then the traditional FR meaning of green and yellow would no longer be in use anywhere: the rule book could be changed so that the homes could be made to show yellow and green with their boringly-conventional main-line meanings. This would allow the banner caution signals to be removed, simplifying the installation. Fortunately, Rhiw Goch is still very much in use, keeping the FR traditional interpretation of yellow and green alive and well, which means that the yellow aspects on the heads at Minffordd have never been brought into use.
See also[edit source]
- Matthews, P (April 2007). "Unusual Signals". IRSE News.