The Decline of The Metro Style Train

[Ridgeblog]

Hey everyone!

When examining newer trains ordered by many TOCs, I noticed there's been a noticeable decline generally in the use of pocket sliding doors; many if nearly all of them use plug doors.

The Aventras all have them, as do the CAF City, Everos and Mk5 coaches.

The only 'new' trains that have pocket doors are the desiro city and Hitachi A Train units, as far as I can see. Why is this the case?

As I understand, pocket sliding doors are useful for high density routes where quick stopping is required, though on the Underground, they also have similar style sliding doors but which open outwards instead of inwards.

Why did many earlier powered door units have pocket sliding doors?

Even on many busy routes such as those on the Overground network or Anglia area, many have been replaced with those with plug doors, mainly the Aventras and CAFs.

Could this be due to cost or management issues, as it cab often be quite hard to find perfect matches or replacements?

 

[yorksrob]

The pocket doors do tend to ruin the window arrangement.

 

[class442]

Is it possible that the functionality of plug door designs have improved over the decades, and as a result pocket doors are no longer necessary?

I find that the doors on Aventras operate quickly, maybe to the point there would be minimal time savings if they had pocket doors instead.

 

[Bletchleyite]

The pocket doors do tend to ruin the window arrangement.

They also cause corrosion as water gets into the pockets.

 

[yorksrob]

Is it possible that the functionality of plug door designs have improved over the decades, and as a result pocket doors are no longer necessary?

I find that the doors on Aventras operate quickly, maybe to the point there would be minimal time savings if they had pocket doors instead.

It certainly seems rare to get issues with them on the 158's (as a passenger who uses them often).

They also cause corrosion as water gets into the pockets.

I must admit, I wan't aware of that.

 

[edwin_m]

The pockets take up internal space for the door itself and for the wall separating the pocket from the interior. The external sliding doors on LU don't need an extra wall but they do reduce the width even in parts of the train that they don't overlap with when open. A plug door can go outside the body profile when it is open.

Pocket doors are also less aerodyamic as the vehicle side can't be smooth. They risk creating draughts as well as rattling when passing another train at speed - IETs get round this by having an inflatable seal, but this is something else to go wrong and I imagine also extends the opening/closing time.

If the door isn't completely smooth (the window may be recessed on one or both sides) there is a hazard if someone puts their hand on it as it is closing, and it is pinched between the recess and the mouth of the pocket.

 

[Ridgeblog]

The pockets take up internal space for the door itself and for the wall separating the pocket from the interior. The external sliding doors on LU don't need an extra wall but they do reduce the width even in parts of the train that they don't overlap with when open. A plug door can go outside the body profile when it is open.

Pocket doors are also less aerodyamic as the vehicle side can't be smooth. They risk creating draughts as well as rattling when passing another train at speed - IETs get round this by having an inflatable seal, but this is something else to go wrong and I imagine also extends the opening/closing time.

If the door isn't completely smooth (the window may be recessed on one or both sides) there is a hazard if someone puts their hand on it as it is closing, and it is pinched between the recess and the mouth of the pocket.

These are very interesting points! I was wondering if technology had something to do with this. So did plug doors become more sophisticated at opening faster, hence their benefits outweigh those of pocket doors, which don't open faster anymore? Also, would the open pockets be harder to maintain or clean due to rusting etc.? Or what about if vacuum pockets were used instead, if that's even possible?

 

[edwin_m]

These are very interesting points! I was wondering if technology had something to do with this. So did plug doors become more sophisticated at opening faster, hence their benefits outweigh those of pocket doors, which don't open faster anymore? Also, would the open pockets be harder to maintain or clean due to rusting etc.? Or what about if vacuum pockets were used instead, if that's even possible?

The sliding plug door is a more complex mechanism, having to move in an L shape, and this creates more scope for failure compared with the linear movement of a pocket door. The swing plug door as used on 158s and 442s is somewhere in between in complexity. I imagine sliding plug doors have gradually improved over time so that most manufacturers now consider they are reliable enough to use.

If you're thinking of creating a vacuum in the pocket to suck the door open, and presumably compressed air to close it, then I doubt that would be practical because of the difficulty sealing the edges of the pocket. Some sliding doors of all types are air-operated, but this is by putting compressed air into a separate cylinder to move a piston which then moves the door.

 

[Sorcerer]

To be honest I'm not quite sure I'd say pocket sliding doors are a characteristic of metro-style trains. You might've seen them there more often but a train can still definitively be a metro train without them unlike something such as fast acceleration and deceleration along with high-capacity walk-through carriages with predominantly longitudinal seating. The S Stock and 2024 stock on the tube are perfect examples of this.

Regardless, and I say this from an outsider perspective, sliding plug doors just seem better than pocket sliding ones for ease of maintenance and replacement. Admittedly this is something I'm not well educated on, but if I had to guess I would say Japanese designs only continue to use them because that's what the manufacturers are used to, and as I understand it Japan is quite protectionist with their rolling stock builders.

 

[Ridgeblog]

To be honest I'm not quite sure I'd say pocket sliding doors are a characteristic of metro-style trains. You might've seen them there more often but a train can still definitively be a metro train without them unlike something such as fast acceleration and deceleration along with high-capacity walk-through carriages with predominantly longitudinal seating. The S Stock and 2024 stock on the tube are perfect examples of this.

Regardless, and I say this from an outsider perspective, sliding plug doors just seem better than pocket sliding ones for ease of maintenance and replacement. Admittedly this is something I'm not well educated on, but if I had to guess I would say Japanese designs only continue to use them because that's what the manufacturers are used to, and as I understand it Japan is quite protectionist with their rolling stock builders.

Yeah, I can definitely see that and it might just be my perspective. It seems pocket doors are outdated now and outward sliding doors are generally the go to for heavy suburban units. I thought express units generally used plug doors because of less opening and stopping as that would wear out the door mechanisms, whereas pocket doors are more durable for that, but I also see even express units have pocket doors, especially newer ones. I did wonder why there were no more orders for 376/378/707/717 style units or if Aventra or anyone else manufactured those style of trains instead of the longer types with heavy seating which everyone else produces.

 

[NCT]

Don't get where the 'decline' comes from. As long as plug doors are considered sufficiently reliable they do have the advantages of not reducing space available for windows, or having to design for doors to open into dual-pane windows.

 

[Ridgeblog]

Don't get where the 'decline' comes from. As long as plug doors are considered sufficiently reliable they do have the advantages of not reducing space available for windows, or having to design for doors to open into dual-pane windows.

Well, I don't really get these benefits because firstly, you can still have windows by making the pockets transparent, and also, I still don't get why pocket doors aren't being used much or improved on, and as for the space, why not design it so that you can still fit a seating area where the pockets are? It just feels to me like express trains, or the design at least, is now being utilised for metro services rather than the other way around. So it's a question of whether it's a worthy tradeoff, ie do plug doors really surpass pocket doors in these uses, or was it simply a comprise for say lack of investment or innovation in metro style trains in recent years? Also, don't outward and plug doors wear out or corrode more since they open outwards against the bodyside rather than inwards?

 

[bramling]

Don't get where the 'decline' comes from. As long as plug doors are considered sufficiently reliable they do have the advantages of not reducing space available for windows, or having to design for doors to open into dual-pane windows.

I guess the “decline” is that there’s been several types of train over recent years which seem to have fairly generic layouts which cater for or have ended up deployed on metro, suburban and mainline work, whilst not being the best fit for any.

Just a few off the top of my head
377 / 387
350
450
458
465
700 / 717
730

In fact this isn’t an entirely new thing, as arguably same could be said for the Mk3 based EMUs, albeit when they were new there was less distinction between the groups, with people still expecting a certain level of provision even on metro services.

I’ve always found it quite surprising that Southern has for many years used their Electrostars indiscriminately, but it seems to work satisfactorily.

 

[py_megapixel]

They risk creating draughts as well as rattling when passing another train at speed - IETs get round this by having an inflatable seal, but this is something else to go wrong and I imagine also extends the opening/closing time.

I think you might be thinking of the Voyagers there, which have plug doors with inflatable seals. The IETs instead have a motorised contraption that pushes the whole door leaf outwards against the frame (which of course would only be possible with a pocket door).

I don't think either of these mechanisms has too much impact on opening/closing times as they engage/disengage while the train is still moving. You can hear them on both types of stock near the start of an acceleration or the end of deceleration - on IETs, a loud clunk as each door moves outwards or inwards, and on Voyagers, a change in how the engine sounds from the inside, as the seal seems to be a much more effective accoustic barrier when inflated than deflated.

I believe the BR Mk3-derived MUs have nothing holding the doors in place except the air pressure in the door mechanism - a fast train passing is often enough to at least partially overcome this, causing the doors to bang loudly.

 

[edwin_m]

Well, I don't really get these benefits because firstly, you can still have windows by making the pockets transparent, and also, I still don't get why pocket doors aren't being used much or improved on, and as for the space, why not design it so that you can still fit a seating area where the pockets are? It just feels to me like express trains, or the design at least, is now being utilised for metro services rather than the other way around. So it's a question of whether it's a worthy tradeoff, ie do plug doors really surpass pocket doors in these uses, or was it simply a comprise for say lack of investment or innovation in metro style trains in recent years? Also, don't outward and plug doors wear out or corrode more since they open outwards against the bodyside rather than inwards?

Older LU stock before had windows in door pockets before they adopted sliding doors on external runners, but this means opening the windows up to clean them, or just leaving them looking dirty. There can indeed be a seating area alongside the pockets, but the reduction in width on Class 150 is enough that nobody has tried putting 3+2 seating there.

It's probably now the case, with IET units introduced and Mk3-based units disappearing, that the majority of the plug doors on the UK network are on express units. Light rail vehicles mostly have plug doors too, so they can't really be considered a design that is optimal for longer-distance trains. As mentioned above, pocket doors also have corrosion issues.

Another hazard with pocket doors is that something like a trapped hand may not be visible down the train because the doors are recessed.

I think you might be thinking of the Voyagers there, which have plug doors with inflatable seals. The IETs instead have a motorised contraption that pushes the whole door leaf outwards against the frame (which of course would only be possible with a pocket door).

I don't think either of these mechanisms has too much impact on opening/closing times as they engage/disengage while the train is still moving. You can hear them on both types of stock near the start of an acceleration or the end of deceleration - on IETs, a loud clunk as each door moves outwards or inwards, and on Voyagers, a change in how the engine sounds from the inside, as the seal seems to be a much more effective accoustic barrier when inflated than deflated.

I believe the BR Mk3-derived MUs have nothing holding the doors in place except the air pressure in the door mechanism - a fast train passing is often enough to at least partially overcome this, causing the doors to bang loudly.

Thanks for the comments and correction. I believe Hitachi based their design on Japanese practice, and one purpose of this mechanism is to reduce pressure pulses when passing trains at speed. Perhaps I made the mental leap to thinking they used air pressure to operate.

 

[Russel]

They risk creating draughts as well as rattling when passing another train at speed - IETs get round this by having an inflatable seal, but this is something else to go wrong and I imagine also extends the opening/closing time.

As said above, it's the Voyagers that have this and I'd assume the 222s as well, it inflates and deflates when the train hits 3mph from memory, it's always obvious from the wind noise in the vestibule when the seal has failed, though to give credit to the build quality of the Voyagers, in 20+ years, I've only encountered this a couple of times!

 

[MCR247]

As said above, it's the Voyagers that have this and I'd assume the 222s as well, it inflates and deflates when the train hits 3mph from memory, it's always obvious from the wind noise in the vestibule when the seal has failed, though to give credit to the build quality of the Voyagers, in 20+ years, I've only encountered this a couple of times!

It’s noticeable when you’re waiting at the door when the trains pulling in and suddenly everything becomes much louder

 

[hwl]

Is it possible that the functionality of plug door designs have improved over the decades, and as a result pocket doors are no longer necessary?

I find that the doors on Aventras operate quickly, maybe to the point there would be minimal time savings if they had pocket doors instead.

Correct.
The change from pneumatic to electrical actuation of door movement have really helped improve opening and closing speeds especially for plug doors (and also improvements in reliability).

 

[edwin_m]

Correct.
The change from pneumatic to electrical actuation of door movement have really helped improve opening and closing speeds especially for plug doors (and also improvements in reliability).

Also the swing plug door, as used on 442s and 158s, hasn't been perpetuated. This was a developmental dead end in my view, very slow-acting and with several features that might risk trapping.

 

[Ridgeblog]

Correct.
The change from pneumatic to electrical actuation of door movement have really helped improve opening and closing speeds especially for plug doors (and also improvements in reliability).

Oh interesting, so does this also apply to swing plug doors as well, or do these have mechanisms more between the functionality of plug doors and sliding slit doors in that they open at a wide angle but have a slit to slide across outwards?

== Doublepost prevention - post automatically merged: 1 Nov 2025 ==

You've got it! This is the main reason why I made this thread as it really struck me how seemingly unsuited these classes are especially when running on high density routes, and it made me wonder if it contributed in any way to overcrowding and punctuality of train services of some tocs when dealing with lots commuters at busy stations. Even if it's just a small difference between opening times of plug doors vs pocket, it still adds up in total and can end up having long term impact on performance and reliability of services on high density routes.

 

[NCT]

On Overground routes with a mix allocation of 378s and 710s I highly doubt there's a discernible difference in door opening and closing times.

 

[frankmoh]

On Overground routes with a mix allocation of 378s and 710s I highly doubt there's a discernible difference in door opening and closing times.

I think the actual door opening time is roughly the same (0.5-1s more?) but the 710s seem to wait a bit longer before letting passengers open doors compared to a 378.

 

[Ridgeblog]

I think the actual door opening time is roughly the same (0.5-1s more?) but the 710s seem to wait a bit longer before letting passengers open doors compared to a 378.

I get that. I'm not saying it can't work, and maybe once in a while you can deploy an express style train on high density metro routes. However, this is a consistent trend, as before that, there were 172s on the GOBLIN line which also has hundreds of commuters per work day, almost like the North London and Lee Valley lines. I mean, I guess if it isn't that effective, then they wouldn't do it, but I still think it's worth questioning whether how much it solves overcrowding problems.

 

[Sorcerer]

I get that. I'm not saying it can't work, and maybe once in a while you can deploy an express style train on high density metro routes. However, this is a consistent trend, as before that, there were 172s on the GOBLIN line which also has hundreds of commuters per work day, almost like the North London and Lee Valley lines. I mean, I guess if it isn't that effective, then they wouldn't do it, but I still think it's worth questioning whether how much it solves overcrowding problems.

The swinging plug doors were the least of the Turbostar's problems on the Gospel Oak to Barking Line. The issue with them was that they were poor accelerators and far too short, not that their door design wasn't what you might call metro-style. When it comes to door design and overcrowding, nowadays it'd mostly be relating to the layout, which in the case of a high capacity metro would require at least three pairs on each side regardless of being swing plug or pocket sliding.

 

[edwin_m]

The swinging plug doors were the least of the Turbostar's problems on the Gospel Oak to Barking Line. The issue with them was that they were poor accelerators and far too short, not that their door design wasn't what you might call metro-style. When it comes to door design and overcrowding, nowadays it'd mostly be relating to the layout, which in the case of a high capacity metro would require at least three pairs on each side regardless of being swing plug or pocket sliding.

To clarify, the doors on Turbostars are Sliding Plug - driven by a sliding actuator which moves the door out of the aperture and slides it sideways in one movement. Swing plug doors are driven by arms off a rotating vertical shaft, as in 442s where the shaft is exposed and 158s where it isn't.

 

[Bletchleyite]

To clarify, the doors on Turbostars are Sliding Plug - driven by a sliding actuator which moves the door out of the aperture and slides it sideways in one movement. Swing plug doors are driven by arms off a rotating vertical shaft, as in 442s where the shaft is exposed and 158s where it isn't.

The Dutch seem to favour this design - apart from some very new kit almost all Dutch trains have this sort of mechanism. The main downsides seem to be that the shaft itself might be a safety issue when it rotates if people are holding it (it provides a natural handrail) and that the doors can only be quite narrow as were they wider they would swing out too far and knock people on the platform over as they opened. The upside is that it's a very simple design with few failure modes.

 

[Sorcerer]

To clarify, the doors on Turbostars are Sliding Plug - driven by a sliding actuator which moves the door out of the aperture and slides it sideways in one movement. Swing plug doors are driven by arms off a rotating vertical shaft, as in 442s where the shaft is exposed and 158s where it isn't.

Thanks for the clarification. I must admit to using the terms "sliding" and "swinging" interchangeably when talking about plug doors as if they were one and the same without putting much thought into it, but in future I might just have to refer to them as plug doors if I'm not sure which kind.

 

[tfw756rider]

The swinging plug doors were the least of the Turbostar's problems on the Gospel Oak to Barking Line. The issue with them was that they were poor accelerators

Nothing against you, but in what way were the 172s poor accelerators? With their efficient 6-speed ZF bus-style mechanical gearboxes (similar ones of which would later be used on the NI C4Ks, and then, back in Britain, the 195s/196s/197s), it has been said on this site that they brought an "electric-like" diesel service to the Snow Hill lines - did they just not have the same effect on the GOBLIN?

 

[Silent]

The pocket doors do tend to ruin the window arrangement.

I think plug doors look better. Thameslink trains look like network southeast era trains with the window and door arrangement whilst 378s remind me of older tube trains

 

[Sorcerer]

Nothing against you, but in what way were the 172s poor accelerators? With their efficient 6-speed ZF bus-style mechanical gearboxes (similar ones of which would later be used on the NI C4Ks, and then, back in Britain, the 195s/196s/197s), it has been said on this site that they brought an "electric-like" diesel service to the Snow Hill lines - did they just not have the same effect on the GOBLIN?

I couldn't find specific numbers but from what I have heard the 172 has an acceleration rate around that of the 170s and 185s (so ~0.49 to 0.50 m/s2) which would be a 0-60mph of about 53/54 seconds. Not bad for a unit running long distances, especially one that's a DMU, but it's not exactly suitable for an urban rail or high-capacity metro system regardless of door design.

For comparison the 345 has an acceleration rate up to 1m/s2 (0-60 of about 27 seconds) and the S Stock has one of up to 1.3m/s2 (0-60 of about 21 seconds). I have heard that this only saves about 10-15 seconds per station stop elsewhere on the forums, but it adds up across the length of the journey. For urban rail systems, 172s are relatively poor accelerators compared to electric traction.

In relation to the original question, this has a much bigger impact than having a plug door on the rolling stock. If there was solid evidence that sliding pocket doors were significantly faster than plug doors and therefore were better suited to metro systems then I would accept it, but as it stands the acceleration and capacity plays a much bigger role in defining a metro-style train than the door design.

(Just a note here for anyone wondering where I got my calculations from. I used an online converter. I don't want to risk filling this post with too much off-topic clutter but I will show the workings out should forum staff request it. Also m/s2 means metres per second squared for anyone unfamiliar).