A signalling question

[Jack Barry]

On the Bakerloo line trains going north can do a main line shunt and come back south at Lambeth North/Piccadilly Circus/Paddington. The signal controlling this move is a colour light but at the Elephant & Castle it is a proper shunt signal.
So my question is why colour light signals instead of shunt signals.
This may also be the case on other lines but I have not checked out yet.

 

[bramling]

On the Bakerloo line trains going north can do a main line shunt and come back south at Lambeth North/Piccadilly Circus/Paddington. The signal controlling this move is a colour light but at the Elephant & Castle it is a proper shunt signal.
So my question is why colour light signals instead of shunt signals.
This may also be the case on other lines but I have not checked out yet.

There doesn’t necessarily have to be a reason other than that the designer felt like doing things in a particular way.

Not sure if there was a change in standards, however shunt signals tended to be used less from the late 1980s onwards - some other schemes reduced their use. JLE, East London Line and Uxbridge branch spring to mind (although not the Central Line).

 

[Jack Barry]

There doesn’t necessarily have to be a reason other than that the designer felt like doing things in a particular way.

Not sure if there was a change in standards, however shunt signals tended to be used less from the late 1980s onwards - some other schemes reduced their use. JLE, East London Line and Uxbridge branch spring to mind (although not the Central Line).

IIRC Elephant & Castle was signalled after the other bits of the Bakerloo and a shunt signal was used, why use a colour light for a shunt move when there is never going to be any passengers on board.

 

[Belperpete]

why use a colour light for a shunt move when there is never going to be any passengers on board.

Because a train can move faster on a main aspect than on a shunt? At an intermediate turn-back, that would clear the platform faster for the following train.

 

[Rogmi]

Stockwell Northern line was another example of a colour light instead of a shunt when reversing N-S.
The crossover was installed in the early 70’s and the existing crossover at Clapham Common was removed.

The crossover was installed in the original C&SLRly platform tunnel and is a longer reversing crossover than is usually found on pre Vic line emergency reversing points. This allows for a faster speed over the crossover and, as far as I can remember, there was never a speed restriction when reversing over the crossover in either direction.

As a general rule of thumb, a green signal means movement at up to line speed, a shunt signal means movement at up to 10mph, often 5. Actual speeds for either type of signal will depend on local instructions. Passengers are not taken past a shunt signal unless the route is secured / proved secured.

As far as Stockwell is concerned, I’m not sure whether a colour signal (the only one on the Northern line when doing a ‘Shunt and Reverse’) was used purely because the longer crossover allowed faster speeds, or because there was a change in the way signalling evolved over the years. From what you’ve said regarding the Bakerloo line shunting, it may be a case of the latter.

 

[Jack Barry]

Stockwell Northern line was another example of a colour light instead of a shunt when reversing N-S.
The crossover was installed in the early 70’s and the existing crossover at Clapham Common was removed.

The crossover was installed in the original C&SLRly platform tunnel and is a longer reversing crossover than is usually found on pre Vic line emergency reversing points. This allows for a faster speed over the crossover and, as far as I can remember, there was never a speed restriction when reversing over the crossover in either direction.

As a general rule of thumb, a green signal means movement at up to line speed, a shunt signal means movement at up to 10mph, often 5. Actual speeds for either type of signal will depend on local instructions. Passengers are not taken past a shunt signal unless the route is secured / proved secured.

As far as Stockwell is concerned, I’m not sure whether a colour signal (the only one on the Northern line when doing a ‘Shunt and Reverse’) was used purely because the longer crossover allowed faster speeds, or because there was a change in the way signalling evolved over the years. From what you’ve said regarding the Bakerloo line shunting, it may be a case of the latter.

That's interesting about Stockwell although why they want to go faster over the crossover I don't know but compared to the Bakerloo crossovers it is further from the station. At Piccadilly Circus the crossover is right at the end of the platform so speed can't be a factor and the crossover has had quite a few derailments over the years, but they have a colour light signal and Paddington not that much different really.

 

[rebmcr]

why they want to go faster over the crossover I don't know

To clear the running lines more quickly.

 

[Jack Barry]

To clear the running lines more quickly.

I understand what your saying as ROGMI said the same for Stockwell but are there not reduced speeds going over such things unless they are built for high speeds and like I said Piccadilly Circus has had lots of problems over the years.

 

[bramling]

Stockwell Northern line was another example of a colour light instead of a shunt when reversing N-S.
The crossover was installed in the early 70’s and the existing crossover at Clapham Common was removed.

The crossover was installed in the original C&SLRly platform tunnel and is a longer reversing crossover than is usually found on pre Vic line emergency reversing points. This allows for a faster speed over the crossover and, as far as I can remember, there was never a speed restriction when reversing over the crossover in either direction.

As a general rule of thumb, a green signal means movement at up to line speed, a shunt signal means movement at up to 10mph, often 5. Actual speeds for either type of signal will depend on local instructions. Passengers are not taken past a shunt signal unless the route is secured / proved secured.

As far as Stockwell is concerned, I’m not sure whether a colour signal (the only one on the Northern line when doing a ‘Shunt and Reverse’) was used purely because the longer crossover allowed faster speeds, or because there was a change in the way signalling evolved over the years. From what you’ve said regarding the Bakerloo line shunting, it may be a case of the latter.

Sorry to spoil what is otherwise a neat theory, however Stockwell crossover is 10 mph! According to the 1981 Northern Line Supplement to Rule Book all crossovers on the line were 15 mph, with no exception shown for Stockwell. At some point this was reduced to 10 mph. One presumes this remains the case under the new signalling. To muddy the waters further, there's facing point locking fitted for the south to north move, but not for the north to south move - so the colour light signal was only ever the equivalent of a shunt signal.

Whether any driver ever *actually* took the crossover at 10 mph is a matter for debate, but that's what the official position seems to have been.

This reminds me of a story where during a project in the 2000s to enhance and standardise speed restriction signage, there was considerable consternation when 15 mph PSR sign appeared at a particular location, with drivers moaning and querying about why such a low speed had suddenly appeared out of the blue, when they'd been used to going over the diverging points concerned at line speed - 35 mph. It turned out this was also covered by the "through all crossovers when diverging from the main line" clause in the line supplement, and therefore the speed limit had actually been *10 mph*!

Believe it or not, there's still a few places on LU where staff don't actually know what the speed limit is.

 

[Jack Barry]

Sorry to spoil what is otherwise a neat theory, however Stockwell crossover is 10 mph! According to the 1981 Northern Line Supplement to Rule Book all crossovers on the line were 15 mph, with no exception shown for Stockwell. At some point this was reduced to 10 mph. One presumes this remains the case under the new signalling. To muddy the waters further, there's facing point locking fitted for the south to north move, but not for the north to south move - so the colour light signal was only ever the equivalent of a shunt signal.

Whether any driver ever *actually* took the crossover at 10 mph is a matter for debate, but that's what the official position seems to have been.

This reminds me of a story where during a project in the 2000s to enhance and standardise speed restriction signage, there was considerable consternation when 15 mph PSR sign appeared at a particular location, with drivers moaning and querying about why such a low speed had suddenly appeared out of the blue, when they'd been used to going over the diverging points concerned at line speed - 35 mph. It turned out this was also covered by the "through all crossovers when diverging from the main line" clause in the line supplement, and therefore the speed limit had actually been *10 mph*!

Believe it or not, there's still a few places on LU where staff don't actually know what the speed limit is.

Well that puts the cat amongst the pigeons as to why have colour light signal although like you say the points were not set for passenger moves when reversing N-S. Is the reaspn that some staff don't know what the actual limit is down to bad training or a management issue.

 

[Jack Barry]

Just a thought perhaps the colour lights was something that was specified in the package for the resignalling.