Riding the Yellows

Railway Signalling and Operations

The "Riding the Yellows" Problem

Mark D. Bej


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"Riding the yellows" is the practice wherein one train proceeds closely behind another at approximately the same speed. With sufficiently close spacing and consistent signal spacing, the rear train will receive yellow after yellow signal. This practice was at least in part due to the lack of foresight in the wording (and implementation) of the rule for the Approach signal, which was:

Name: Approach.
Indication: Approach next signal prepared to stop.

           50mph>                50mph>
-----------#####>----------------#####>-----------------------------------------
       |--R       |--Y       |--R       |--G       |--G       |--Y       |--R

               50mph>                50mph>
---------------#####>----------------#####>-------------------------------------
       |--R       |--R       |--R       |--R       |--G       |--Y       |--R

                   50mph>                50mph>
-------------------#####>----------------#####>---------------------------------
       |--Y       |--R       |--Y       |--R       |--G       |--Y       |--R

                        50mph>                50mph>
------------------------#####>----------------#####>----------------------------
       |--Y       |--R       |--Y       |--R       |--G       |--Y       |--R

                            50mph>                50mph>
----------------------------#####>----------------#####>------------------------
       |--Y       |--R       |--Y       |--R       |--R       |--Y       |--R

                                50mph>                50mph>
--------------------------------#####>----------------#####>--------------------
       |--G       |--Y       |--R       |--Y       |--R       |--Y       |--R

The engineer of the rear train "knows" there is a train in front of him, as he has not gotten a red signal. (Of course, that engineer is inferring the presence of that train, unless he is in direct contact with it by radio or has some other 'meta-level' knowledge. The act of inference itself is not proper, as it may lead to incorrect actions, as the reader will soon see.)

Under the older version of the Approach rule quoted above, the engineer of the rear train was not required to slow down. Some engineers would become complacent and continue their present speed. In effect, they expected the front train to continue at its speed forever. This is potentially disasterous, as shown below. I will simply continue the sequence above:


                                50mph>                50mph>
--------------------------------#####>----------------#####>--------------------
       |--G       |--Y       |--R       |--Y       |--R       |--Y       |--R

                                    50mph>                50mph>
------------------------------------#####>----------------#####>----------------
       |--G       |--Y       |--R       |--Y       |--R       |--R       |--R

                                      50mph>                40mph>
--------------------------------------#####>----------------#####>--------------
       |--G       |--Y       |--R       |--R       |--R       |--R       |--R

                                          50mph>              30mph>
------------------------------------------#####>--------------#####>------------
       |--G       |--G       |--Y       |--R       |--Y       |--R       |--R

                                              50mph>            20mph>
----------------------------------------------#####>------------#####>----------
       |--G       |--G       |--Y       |--Y       |--Y       |--R       |--R

                                                  50mph>          10mph>
--------------------------------------------------#####>----------#####>--------
       |--G       |--G       |--Y       |--Y       |--R       |--R       |--R

                                                      50mph>         STOP>
------------------------------------------------------#####>--------#####>------
       |--G       |--G       |--R       |--Y       |--R       |--R       |--R

At 'ideal' (worst-scenario) spacing, the rear train will continue its habit of passing yellow signals at speed, then suddenly find a red signal and be unable to stop. At slightly more than worst-scenario spacing, the rear train will observe the second signal in the rear of the front train dropping from yellow to red and will have about one braking distance to stop.

As a result, the clause demanding Medium Speed after a yellow signal was added. The rule thus reads

Name: Approach.
Indication: Approach next signal prepared to stop. Train exceeding Medium Speed must at once reduce to that speed.
Adhering to this rule yields a considerably different result:

           50mph>                50mph>
-----------#####>----------------#####>-----------------------------------------
       |--R       |--Y       |--R       |--G       |--G       |--Y       |--R

               30mph>                50mph>
---------------#####>----------------#####>-------------------------------------
       |--R       |--R       |--R       |--R       |--G       |--Y       |--R

                 30mph>                  50mph>
-----------------#####>------------------#####>---------------------------------
       |--Y       |--R       |--Y       |--R       |--G       |--Y       |--R

                    30mph>                    50mph>
--------------------#####>--------------------#####>----------------------------
       |--Y       |--R       |--Y       |--R       |--G       |--Y       |--R

                      30mph>                      50mph>
----------------------#####>----------------------#####>------------------------
       |--Y       |--R       |--Y       |--R       |--R       |--Y       |--R

                        30mph>                        50mph>
------------------------#####>------------------------#####>--------------------
       |--Y       |--R       |--G       |--Y       |--R       |--Y       |--R

                           40mph>                         40mph>
---------------------------#####>-------------------------#####>----------------
       |--G       |--Y       |--R       |--Y       |--R       |--Y       |--R

And now, seeing a green rather than yellow signal, the engineer of the rear train can surmise that, by following the rules, he has been spaced back half a block. Note that a safe stop ensues:


                           40mph>                         40mph>
---------------------------#####>-------------------------#####>----------------
       |--G       |--Y       |--R       |--Y       |--R       |--Y       |--R

                              50mph>                        30mph>
------------------------------#####>------------------------#####>--------------
       |--G       |--Y       |--R       |--Y       |--R       |--Y       |--R

                                  50mph>                      30mph>
----------------------------------#####>----------------------#####>------------
       |--G       |--Y       |--R       |--G       |--Y       |--R       |--R

                                      50mph>                    20mph>
--------------------------------------#####>--------------------#####>----------
       |--G       |--Y       |--R       |--R       |--Y       |--R       |--R

                                          50mph>                  10mph>
------------------------------------------#####>------------------#####>--------
       |--G       |--G       |--Y       |--R       |--Y       |--R       |--R

                                              40mph>                 STOP>
----------------------------------------------#####>----------------#####>------
       |--G       |--G       |--Y       |--R       |--Y       |--R       |--R
Someone, previously, wrote:

> A couple of years ago Alan Williams, in his column in Modern Railways, was
> going on about a resignalling project done badly. The line was 3 aspect,
> and the signal spacing was extremely large because of the freight traffic
> that used it. As a result, EMUs were passing yellows and maintaining full
> speed, confident that they could stop if the next signal was red.
Given the wording, I assume this was in Britain or Australia. Would the author email me please?