Requirements for 4 aspect signalling

[Class172]

I realise that this question may have a number of answers due to it being based upon variable factors: line speed; frequency of trains etc. But are there any general requirements that need to be fulfilled in order for a line to have 4 aspect signalling, as opposed to 3 aspect.

The thought popped into my head when passing through Kidderminster the other week. Now I had been to Leicester earlier that year and noticed the signals there were 3 aspect LEDs. Then following the Stourbridge-Hartlebury resignalling, I noticed that although it had 3 aspect LEDs similar to Leicester, there was a gap above the main aspect with room for a second yellow aspect (ie 4 aspect signal). Is this because provision has been made for future conversion to 4 aspect signalling on the line, such as when it satisfies a possible criterion? The intermediate block signals at Droitwich also have extra room for another aspect so is this linked also?

Here are two pictures below showing the two locations and the signals I am referring to.

Naturally I would have expected Leicester to be given 4 aspect signals first, but it seems this isn't true since Leicester was fairly recently resignalled (?). Could anyone explain also why this seems to be the case - what is needed for 4 aspect signalling.

 

[Tomnick]

I can't really comment on the criteria for providing 4-aspect signalling (if any exist), other than to suggest that there's not much justification for providing something that the traffic levels don't demand! Leicester wasn't resignalled though - the existing layout and signalling was just recontrolled to the EMCC. I think the resignalling proper comes in the next couple of years!

 

[Railsigns]

As you've rightly guessed, the decision to go for 4-aspect signalling is mainly influenced by speed (and therefore braking distance) and frequency of trains. Cost is a factor that prevents its more widespread application.

Dorman LED signals, as seen in both of your photos, are available with either 'long' or 'short' housings. At first, the long housing was favoured, as it was felt that drivers would be less likely to run past an unlit signal in the event of failure, if it was of similar size and appearance to a traditional multi-lens signal head. Use of the tall housing doesn't imply any intention of adding a double yellow aspect to the signal in the future.

 

[Class172]

Leicester wasn't resignalled though - the existing layout and signalling was just recontrolled to the EMCC. I think the resignalling proper comes in the next couple of years!

I wasn't sure if it was recent or not but the signals looked fairly new, hence the '?'.

As you've rightly guessed, the decision to go for 4-aspect signalling is mainly influenced by speed (and therefore braking distance) and frequency of trains. Cost is a factor that prevents its more widespread application.

Dorman LED signals, as seen in both of your photos, are available with either 'long' or 'short' housings. At first, the long housing was favoured, as it was felt that drivers would be less likely to run past an unlit signal in the event of failure, if it was of similar size and appearance to a traditional multi-lens signal head. Use of the tall housing doesn't imply any intention of adding a double yellow aspect to the signal in the future.

Thank you for clarifying this - I suspected speed and frequency were the main reasons. I wasn't aware sighting was the reason for the long housing on the dorman LED signals, though I can understand the reasoning, as I prefer the look of the large housing signals, being more alike the traditional colour signals. That would also explain my suspicions relating to future-proofing.

 

[Tomnick]

I wasn't sure if it was recent or not but the signals looked fairly new, hence the '?'.

There hasn't been any resignalling - that was my point! Most (all?) of the signals have been replaced by equivalent LED signals, but there have been no significant changes on the ground.

 

[Railsigns]

Leicester was last resignalled in 1986, although signal head replacement and a re-control to East Midlands CC have been carried out since then.

 

[Cherry_Picker]

It used to be four aspect going into Kiddy until the recent resignalling. It still feels odd going into the station with a terminating service on a single yellow.

 

[ChiefPlanner]

Very roughly 4 aspect is ideal for very busy and fast main lines , and also (with closer signalling positions for even busier suburban lines - think Charing Cross to Lewisham where you need to keep trains moving at say 2 min or 90 second headways at a lower line speed)

3 aspect is a sort of compromise where you may get say up to 10 tph - but you also need to keep traffic moving on restrictive aspects - a good example being Basingstoke to Reading where there is a mix of traffic (some fast up to and above 75mph and stopping DMU trains) - which was resignalled not so long with an upgrade from 2 to 3 aspect. The bonus was that class 4 Freight and Cross Country trains could move on quicker with some confirmation of what was ahead.

There is a booklet produced eons ago by the IRSE on "Signalling the Layout" which explains some of the principles.