Questions regarding Dartford

[Jack D]

Hi all,

Can anyone explain why there is a strange short siding at Dartford. It's too small to hold anything so I'm just wandering what its purpose is.



Additionally, does anyone know why it has platforms 3a and 4a when there are only four platforms in total?



Thanks in advance

 

[brad465]

That short "siding" is a set of trap points, designed to stop a runaway from getting onto the mainline, where it can cause more serious disruption/damage. Every siding/loop has some form of trap/catch points on exit, with the points set to derail by default, only becoming clear when the route is set by a signaller.

As for the "a" platforms, presumably Dartford has the capability of holding trains in separate parts of the same platform, either both in different services, or one short-length service focused at one end, so the platform announcements/displays say the service is in "a" or "b". I saw presumably because I don't believe this is commonly practised at Dartford, unlike stations such as Tonbridge or Ashford, where a Southern service and a Southeastern service often occupy the same numbered platform simultaneously, but in different letter divisions.

 

[Jack D]

Ah I see. Thanks!

 

[Kite159]

Maybe the 'A' part of the platforms have been used in the past when a longer train (say an uncommon 12 coach set) comes in and divides with 8 heading back towards London and 4 going to the sidings/depot until afternoon peak.

 

[waverley47]

That short "siding" is a set of trap points, designed to stop a runaway from getting onto the mainline, where it can cause more serious disruption/damage. Every siding/loop has some form of trap/catch points on exit, with the points set to derail by default, only becoming clear when the route is set by a signaller.

As for the "a" platforms, presumably Dartford has the capability of holding trains in separate parts of the same platform, either both in different services, or one short-length service focused at one end, so the platform announcements/displays say the service is in "a" or "b". I saw presumably because I don't believe this is commonly practised at Dartford, unlike stations such as Tonbridge or Ashford, where a Southern service and a Southeastern service often occupy the same numbered platform simultaneously, but in different letter divisions.

I might have to check your working on the first point.

I think in this particular example of the up island at Dartford, those points are overlap protection rather than trap points.

eg; being able to signal a train straight from the up line into platform 1, with a stationary train signalled out of platform 2 towards London. I don't know if there's a ' warner route' into platforms 1 or 2 but if not then I'd hesitate it has a dual use.

 

[Class15]

That short "siding" is a set of trap points,

It looks a bit long for trap points to be honest? The trap points at the bay platform at Gospel Oak, for example, are literally two tiny pieces of rail without buffers or any real pointwork.

 

[brad465]

I might have to check your working on the first point.

I think in this particular example of the up island at Dartford, those points are overlap protection rather than trap points.

eg; being able to signal a train straight from the up line into platform 1, with a stationary train signalled out of platform 2 towards London. I don't know if there's a ' warner route' into platforms 1 or 2 but if not then I'd hesitate it has a dual use.

Useful to know. Until you said this I thought it was out of the furthest carriage siding, when actually it's from P1. It probably does though double up though for preventing runaways entering the mainline unauthorised.

I'm guessing the same "short siding" clearly visible on the Tanners Hill bank serves a similar purpose: allow an up train from Lewisham to enter the descending bank on a red, while another service that avoided Lewisham is using the Up Fast. This is particularly important as the steep gradient increases stopping distance.

 

[John Webb]

It looks a bit long for trap points to be honest? The trap points at the bay platform at Gospel Oak, for example, are literally two tiny pieces of rail without buffers or any real pointwork.

It is a trap point, but because it serves a platform which trains enter from the other end with some speed it needs to be longer. The buffer stop is clamped to the rails in such a way that it absorbs the energy in the train by sliding along the track which extends some feet beyond it.
The Gospel Oak bay platform on the other hand can only be left by a train that is at rest and which will only be doing a very low speed so the short trap point there is all that's needed to protect the running line from an incorrect move.

 

[Class15]

It is a trap point, but because it serves a platform which trains enter from the other end with some speed it needs to be longer. The buffer stop is clamped to the rails in such a way that it absorbs the energy in the train by sliding along the track which extends some feet beyond it.
The Gospel Oak bay platform on the other hand can only be left by a train that is at rest and which will only be doing a very low speed so the short trap point there is all that's needed to protect the running line from an incorrect move.

Thank you, that makes a lot of sense.

 

[Peter Mugridge]

It is a trap point, but because it serves a platform which trains enter from the other end with some speed it needs to be longer. The buffer stop is clamped to the rails in such a way that it absorbs the energy in the train by sliding along the track which extends some feet beyond it.
The Gospel Oak bay platform on the other hand can only be left by a train that is at rest and which will only be doing a very low speed so the short trap point there is all that's needed to protect the running line from an incorrect move.

I'm guessing it has a buffer partly because of the higher speed risk you mention, but also to protect what's behind it in the event it ever gets used?

 

[Jack D]

How often are they used? Not too often I would hope…

 

[brad465]

How often are they used? Not too often I would hope…

Very rare for any example to be used, but you don't have to go back far in time to find an example of where trap points were used to derail something. West Ealing in April last year is the most recent example that comes to mind involving a freight train.

 

[zwk500]

It is a trap point, but because it serves a platform which trains enter from the other end with some speed it needs to be longer. The buffer stop is clamped to the rails in such a way that it absorbs the energy in the train by sliding along the track which extends some feet beyond it.
The Gospel Oak bay platform on the other hand can only be left by a train that is at rest and which will only be doing a very low speed so the short trap point there is all that's needed to protect the running line from an incorrect move.

Are they still trap points if they are only providing an alternative overlap? I always understood trap points only applied at the exit of sidings, yards and depots when protecting the mainline. Appreciate this is splitting hairs.

Every siding/loop has some form of trap/catch points on exit, with the points set to derail by default, only becoming clear when the route is set by a signaller.

I don't believe this is correct. I'm welcome to be overruled, but my understanding is that Catch points are provided on the mainline to derail vehicles running away on steep gradients, while trap points only need to be provided if insufficient overlap on the signal protecting the exit of the sidings. Many loops/sidings/depots/yards/etc have either sufficient distance from the exit signal to provide an overlap that remains clear of the fouling point, or a headshunt, some of which are long enough for entire trains, which provides the function of protecting the mainline but is not a trap point, as trains can be routed in and out as required for shunting. I can certainly think of many passenger-rated loops that have no trap/catch points at either end.

I'm guessing it has a buffer partly because of the higher speed risk you mention, but also to protect what's behind it in the event it ever gets used?

I suspect also because it's providing a passenger-rated route.

 

[edwin_m]

Trap points are generally not used on passenger lines, because the risk of inadvertent derailment is considered to outweigh the smaller risk of preventing a more serious collision. They are indeed used on the exit from depots, sidings and yards, where there are no passengers but there may be other hazards such as wagons left unbraked that might otherwise roll out onto the main line. If the layout of the depot/siding/yard means there is a straight ahead route that would prevent a train rolling onto the main line, then the points controlling that move will be worked with the ones controlling the main line connection, so that an errant train is always diverted away from the main line unless one is intended to be arriving or departing. This is known as trapping even if there is no actual trap point involved.

There are fairly rare cases where trapping is in fact used on a passenger line, the OP apparently being one of them. As mentioned, the friction buffer stop means an overrunning train will be stopped slightly more gently than if it was just dumped onto the ballast, probably therefore reducing injuries to passengers on board. A friction stop can also have extra shoes behind it so the deceleration increases if it is pushed back far enough - reducing the rate of buildup of acceleration gives passengers a fraction of a second to brace themselves.

 

[frodshamfella]

Years ago when the Dartford services were operated by the slam door sets. It was very common during the off peaks and weekends for trains to be 4 car units on all routes. The Gillingham was normally longer , but the others would usually be 4. It was quite normal for say Platform 4A Charing X via Bexleyheath, and 4B Charing X via Sidcup for example.