3-aspect signalling on the ECML

[rogerzilla]

Watching a cab video on YouTube, I was surprised that the ECML north of Newcastle, or at least some of it, doesn't have 4-aspect signalling.  How common is 4-aspect, and does it only pay off when there are very frequent trains and short block sections?

[phil-b259]

As you surmise 4 aspect signalling is primary there so as to increase the capacity of a given piece of line (just as 3 aspect signalling gives a boost in capacity over two aspect signalling. Four aspect signalling means the driver has in effect two short signal sections to brake from line speed to a stand rather than one long one under 3 aspect signalling.

 

This is expertly demonstrated on the ECML in south where the slow lines have a maximum line speed of 100mph and 3 aspect signalling yet the fast lines which are rated at 125mph get 4 aspect signalling as the spacing is such that its impossible to brake to a stand from 125mph in a single signal section.

 

Of course you could make each signal section longer and then 3 aspects with 125mph running is fine - but that will reduce capacity.

 

However the more aspects you have to more signalling equipment you need (most of which you don't see as its in lineside cabinets or relay rooms) so the signalling engineers will tailor what is provided to what the operations department needs -  and nothing more.

 

Therefore because the ECML north of Newcastle is much less busy that south of there, 3 aspect is fine.

[St. Simon]

1 hour ago, rogerzilla said:

Watching a cab video on YouTube, I was surprised that the ECML north of Newcastle, or at least some of it, doesn't have 4-aspect signalling.  How common is 4-aspect, and does it only pay off when there are very frequent trains and short block sections?

Hi,

 

If 3 aspect signalling allows the capacity you need (both now and in the future) at the braking distance, then why go for four aspect?

 

It is a common myth in the hobby that the number of aspects is dictated by the importance of the line, but it’s not true. You start by calculating the headway given by each set of aspect signalling within the given line speed (using the braking distance) and then choose which gives you the better headway than you need.

 

The formula for headway for each aspect signalling is as follows:

 

3 Aspect = [Sighting Distance + (2 x Braking Distance) + Overlap Length] / Line speed

 

4 Aspect = [Sighting Distance + (1.5 x Braking Distance) + Overlap Length] / Line speed.

 

For 2 Aspect, the calculation is different. You get your headway time, multiply it by the line speed. You then take away the Sighting Distance, Braking Distance and Overlap Length from this distance which gives you the distance between the two stop signals. The determination for using either two or three aspects is, as a rule of thump, is that if that distance is greater than 3 x braking distance, than 2-aspect signals is fine, otherwise 3-aspect is better.

 

Simon