Signals - First or Last axle check

[Peter0124]

Hi, is there any way of finding out whether a specific signal is front or back axle controlled, so that when said axle passes the signal the train moves to the next berth on a signal map for example?

Is public data on this available anywhere?

Thanks

 

[edwin_m]

The move of the train description to the next berth will be triggered by the train occupying the first track circuit after the signal, regardless of whether the signal itself is first wheel or last wheel replacement. However, on most automatic signals the approach and overlap are on the same track circuit so neither happens until the front of the train reaches the end of the overlap.

 

[Peter0124]

The move of the train description to the next berth will be triggered by the train occupying the first track circuit after the signal, regardless of whether the signal itself is first wheel or last wheel replacement. However, on most automatic signals the approach and overlap are on the same track circuit so neither happens until the front of the train reaches the end of the overlap.

Do you know if there is any detailed map of the location of track circuits available? Also what is Approach and Overlap if you don't mind me asking. Thanks

 

[edwin_m]

Do you know if there is any detailed map of the location of track circuits available? Also what is Approach and Overlap if you don't mind me asking. Thanks

Track circuits (and provision or otherwise of Last Wheel Replacement) aren't in the public domain as far as I know - companies in the industry have access but probably pay a fee for it.

The overlap is the distance beyond a signal that is kept clear in case a train approaching the signal at red runs past it. A signal will not clear from red unless the line is unoccupied all the way to the end of the overlap of the next signal (give or take some complications that aren't relevant here). "Approach" I'm using a little sloppily to refer to the track circuit the train is occupying when it is just about to pass the signal. It's sometimes known as the berth track circuit, but that terminology causes confusion as "berth" is used to mean something slightly different when you're talking about train descriptions.

 

[MarkyT]

Track circuits (and provision or otherwise of Last Wheel Replacement) aren't in the public domain as far as I know - companies in the industry have access but probably pay a fee for it.

The overlap is the distance beyond a signal that is kept clear in case a train approaching the signal at red runs past it. A signal will not clear from red unless the line is unoccupied all the way to the end of the overlap of the next signal (give or take some complications that aren't relevant here). "Approach" I'm using a little sloppily to refer to the track circuit the train is occupying when it is just about to pass the signal. It's sometimes known as the berth track circuit, but that terminology causes confusion as "berth" is used to mean something slightly different when you're talking about train descriptions.

First wheel replacement has been the default arrangement for decades on new schemes. There are some good reasons for deviating from that on a case-by-case basis. Propelling moves are an example. Early colour light schemes employed it in many places for steam locos and old diesels like class 20s running with boiler or long hood forward. Without last wheel replacement there was a danger the front axle could replace the signal to red before the driver in a cab some 15 to 20m behind passed the signal. Hybrid replacement circuits often delayed replacement until second track circuit occupied to overcome that.

 

[edwin_m]

First wheel replacement has been the default arrangement for decades on new schemes. There are some good reasons for deviating from that on a case-by-case basis. Propelling moves are an example. Early colour light schemes employed it in many places for steam locos and old diesels like class 20s running with boiler or long hood forward. Without last wheel replacement there was a danger the front axle could replace the signal to red before the driver in a cab some 15 to 20m behind passed the signal. Hybrid replacement circuits often delayed replacement until second track circuit occupied to overcome that.

Also, I imagine, in the days when guards were expected to observe signals.

 

[MarkyT]

Also, I imagine, in the days when guards were expected to observe signals.

Longstanding rules require a signaller to keep a signal lever in the off position until the entire train clears all the junctions in the route, so a guard looking out at the rear would see a proceed in the case of a semaphore or early lever-operated colour light, not track circuit replaced, but with TC clearance proved in the initial solenoid lock release for the lever. Engineers also invented sequential locking to ensure signals were replaced to danger before a following train could be dealt with.