The requirements for yellow lines are less clear than they should be. I've spent some time scratching my head at the relevant standard to work out exactly whay we should do in various cases.
In a nutshell, the standard (Rail Industry Standard RIS-7016-INS, issue 2, "Interface between Station Platforms, Track, Trains and Buffer Stops") can be summarised as:
1) Yellow line not required by default:
"The default arrangement shall be yellow tactiles 760 mm from the platform edge" but also "The default arrangement has been found to be the most appropriate for the majority of GB mainline platforms, but there are some platforms where alternative arrangements provide a better overall management of risk."
2) If there is some reason to do so, a yellow line and a non-yellow tactile can be used instead. There are various reasons why you might do this, such as sighting, narrow platforms, or an aerodynamic risk assesssment showing it's needed, then a yellow line will be provided instead as a marker for the safe distance from platform edge, at a distance determined by risk assessment.
Some of the confusing arrangements indicated may be the result of this paragraph too:
"The yellow line can have a beneficial impact on behaviour overall when optimally positioned and clearly identifiable. However, it is considered a weak hazard control, because it relies on passengers to notice and then decide to stay behind it. It is assumed that some passengers will cross the yellow line. Therefore it is good practice to consider additional hazard controls to reinforce or enforce the yellow line, based on the level of risk at the platform."
3) One of the normal reasons for the yellow line being placed behind the tactiles is aerodynamic risk. In particular, if trains pass at 100mph or more, then this applies:
"At a station platform, where the permissible or enhanced permissible speed on the adjacent line is greater than 100 mph (160 km/h), a yellow line shall be provided on the platform, together with warning signs. The yellow line shall be positioned so that people standing immediately behind the line are at least 1500 mm away from the platform edge."
This 1500 can be reduced where this is likely to lead to overcrowding, and other mitigation measures would likely be required, as per risk assessment.
Freight trains also need to be taken into consideration:
At a station platform, where freight trains, including container traffic but excluding those trains with the same aerodynamic profile as passenger trains, such as mail trains, pass, or are proposed to pass, on the adjacent line at speeds greater than 45 mph (75 km/h), it is good practice to take action to reduce the risk from the aerodynamic effects of passing trains to lightweight objects and vulnerable passengers on station platforms, for example, pushchairs, wheelchairs, the elderly or frail.
In terms of the safety of people and wheeled items in stations, the risk from slipstream effects can be mitigated by frequent passenger warning announcements, increased yellow line clearances from the platform edge or even segregated safe zones on the platform.
There's a lot more in the standard about how to determine the ideal yellow line position, and I've only really scratched the surface of all the things to be considered, but that's a reasonable summary.
FYI yellow hatching is a risk assessment measure under 'Managing crowd positions", and is suggetsed to be used at pinch points on platforms to discourage people from congregating in areas where there is little space to do so and/or the risks are higher.
The standard does not mention a red line so I can only guess what that's for, which would be someone decided, whilst doing a risk assessment, that the yellow line wasn't quite appropriate, and they needed something else, hence they chose red. Whether that makes any sense I don't know.
