Priority at junctions

[AdamWW]

The Coryton line is a short single line branch which joins what I shall call the main line between Cardiff and Rhymney.

I've noticed that at the junction, while a train from Coryton will immediately follow a main line train towards Cardiff, the reverse is almost never true.
Looking at realtimetrains shows that very often a train from Coryton will wait at the junction for 5 or 6 minutes, even though it looks as if it should have had plenty of time to get onto the main line without delaying the next train.

Watching Traksy suggests that the reason is that the junction is set for the main line as soon as a train enters the block before the junction - and it takes quite a while to traverse this block (including a station call).

Having a train waiting on the branch leads to knock-on delays, because the next train onto the branch is delayed (often holding up the main line train behind it), and there is a short turnaround at the end of the branch so it's hard for it to make up time = and then on the way back it's likely to find the junction already set against it.

So my question is - why is it done this way?

I can see the reasoning behind setting a route early enough that a driver doesn't get a yellow aspect for a junction and have to slow down unnecessarily. (It looks in fact as if the route is set just as the train enters the block but maybe it's actually before the signal comes in sight).

But given that all passenger trains are scheduled to call at Heath High Level, just before the junction, does that really matter here? Does a driver have to drive much more carefully towards a red signal than a station stop?
Or can the signalling system not cope with setting a route a certain number of minutes after the train has entered the section rather than immediately?

And even if it should normally be done like this, wouldn't it be possible to manually give priority to the branch when one of the trains is running late, trading at worst a short delay to the main line train for a much larger one for the branch line train?

It looks to me as if there ought to be a better way of doing things. I'm sure there's a good reason why there isn't, but I'd be curious to know what that is. The Valley lines service at the moment falls apart very easily, and this doesn't help.

 

[43066]

Does a driver have to drive much more carefully towards a red signal than a station stop?

As a rule, yes. Or certainly more slowly according to most driving policies. For example you might normally enter a platform at 35mph, but be required to do so at max 20mph if there’s a red signal at the end of it.

 

[Pyewipecarshed]

I'm not familiar with the area any more, but I believe it's controlled from the Wales ROC? At the larger signalling/operations centres the signallers are assisted by ARS (Automatic Route Setting) which generally will prioritise trains on the main route.
It is possible to override ARS but most signallers, particularly newer ones, prefer to let it do its thing.

 

[EveningStarr]

Worth noting too, that on the Down line from Rhymney, the section for Heath High Level goes all the way from Lisvane station. I.e, there's a signal at the bottom end of LVT platform that covers the section all the way to the bottom end of HHL, meaning Llanishen station and Heath High Level station are in the same section. There is a repeater signal just after the bridge at the south of Llanishen, which shows the aspect of the signal at HHL, allowing the driver to know if to approach it as red or green.

My guess is that it's an Automatic Routing System timing rule - the Down Coryton train (towards Penarth) is supposed to be ahead of the Down Rhymney train (towards Barry Island or Bridgend), but if the Penarth is slightly late, as it often is due to the tight schedule on that branch, the route is automatically set for the Barry, regardless of how tiny the delay. Like I say, I'm not sure how accurate that is, but that would be my guess. It's an incredibly frustrating situation, leading to unnecessary delays and alterations to calling patterns.

 

[AdamWW]

Worth noting too, that on the Down line from Rhymney, the section for Heath High Level goes all the way from Lisvane station. I.e, there's a signal at the bottom end of LVT platform that covers the section all the way to the bottom end of HHL, meaning Llanishen station and Heath High Level station are in the same section. There is a repeater signal just after the bridge at the south of Llanishen, which shows the aspect of the signal at HHL, allowing the driver to know if to approach it as red or green.

Interesting. I was wondering just where the signals were.

My guess is that it's an Automatic Routing System timing rule - the Down Coryton train (towards Penarth) is supposed to be ahead of the Down Rhymney train (towards Barry Island or Bridgend), but if the Penarth is slightly late, as it often is due to the tight schedule on that branch, the route is automatically set for the Barry, regardless of how tiny the delay. Like I say, I'm not sure how accurate that is, but that would be my guess.

It doesn't seem to me as if it's doing anything as sophisticated as setting routes based on the timetable.

At any rate Traksy seems to show that the route gets set for the main line as soon as a down train (i.e Cardiff bound) enters the block before the junction.

Trains from Coryton certainly don't get the junction set for them if they arrive on time and the preceding down train on the main line is late but has already entered that block.

Even if the rule was that main line trains always take priority, it looks as if a train coming off the branch shouldn't have to wait more than a couple of minutes (which it can then make up due to slack in the timetable up to Queen Street). Instead they have to wait up to around 6 minutes - multiply by 6 down trains per hour on the main line, add in having to cross up trains, and there aren't that many minutes in a given hour that a train can make it off the branch.

It's an incredibly frustrating situation, leading to unnecessary delays and alterations to calling patterns.

And the "alteration" is usually chucking everyone out at Ty Glas to wait for the next one. Assuming that isn't turned back too.

It does seem frustratingly unnecessary, but there may be an excellent reason that I'm not aware of that it just has to be done this way, hence my question.
I think there is resignalling planned and I would hope that even if it does have to be this way at the moment, things could be improved with a different signalling setup.

But it would be lovely if it could be done within the constraints of the existing signalling system.