Intercity-style stock demonstrates a complete disconnect with the point of the route -
A commuter route needs commuter stock. It isn’t the 1980s anymore.
Single doors - even wide ones, and enclosed vestibules are not suitable for the high passenger churn at major stations.
The tail cannot wag the dog. End to end / long distance passengers make up a fraction of the users of the route.
So, to stop the tail wagging the dog, you would cut the tail off the dog. Longer-distance travel is more likely to be discressionary travel than commuting is, and of course you are on the train for longer, so for a long-distance passenger it is more important for the train to be comfortable. You say the route is a commuter route and needs commuter stock, if so then forget it, just scrap the route and replace it with a buntch of seperate services like Cardiff-Bristol, Bristol-Salisbury, Salisbury-Southampton and Southampton-Portsmouth, allowing you to use EMUs on the last leg (and hopefully the first before too long). I would very much prefer however that the rail industry tries to attract modal shift away from cars, particularly those doing longer trips, by providing something close to intercity-style stock (the kitchen/buffet is probably not justified, but the rest ought to be provided).
The argument that "Single doors - even wide ones, and enclosed vestibules are not suitable for the high passenger churn at major stations." is valid to a degree but not as strong as you make it appear. Cardiff Central, Newport, Reading, Bath, York and Stockport are all major stations (to give just a few examples) and yet intercity trains call at all of them on a regular basis. You simply have to ensure the timetable allows for the longer dwell time of such stock rather than timing everything for suburban units and making the intercity and regional trains run late.
If successful, that would bring the balance of passengers back up to a more even mix of short-distance commuters and longer-distances travellers (I still don't expect that many doing Cardiff-Portsmouth throughout, but there will be some and also many doing things like Cardiff to Salisbury and Portsmouth to Bath), meaning it would no longer be 'the tail wagging the dog'.
NO NO NO
A 1/3 2/3 unit with gangways (CAF 196 for example) would be ideal for Cardiff to Portsmouth. Only if you ignore longer-distance passengers. The 1/3 2/3 doors keep dwell times down yes, but they reduce toilet privacy (which bothers my grandmother) and, without enclosed vestibules, they disrupt the saloon atmosphere at every stop, not fun if doing a long-distance trip late on a winter evening and give a bit of extra standing room for the peaks trains are not TARDISes, that 'extra standing room' has to come from somewhere - compare a 197 to a 175 and that somewhere is from legroom, luggage space, toilets and tables - all of those being things long-distance passengers want. A modern day Chiltern 168 with end gangways and better acceleration due to the ZF gearbox. For all the eco warriors fit the Hybrid MTU power pack. In the future when battery density has improved very easy to replace fuel tank and engine/gearbox raft with battery/motor replacement. How do you charge the batteries? I agree the acceleration of a 196/197 would be helpful on the Cardiff-Portsmouth route (it would help make up for the dwell times caused by using narrow doors and preserving toilet privacy by having one set of doors at/near the vehicle end) but the 196/197 is still a diesel-mechanical unit presumably with no traction power cabling. All new diesel units need to be either be bi-mode/tri-mode or, at the very least, diesel-electric with passive provision for convertion to bi-mode/tri-mode. Anything else is just giving DfT/treasury an excuse not to electrify.
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A 5-car version wouldn't need end gangways because the platforms at intermediate stations are limited to that length anyway.
Class 444s are 5-car and have gangways which are still useful. Platforms can be extended and rolling stock can be cascaded. While a new 5-car unit for Cardiff-Portsmouth wouldn't need gangways on day one, it's prudent to include them to allow for the future. Personally, I think all new multiple units with a top speed of 110mph or less should have unit end gangways fitted unless they are so long that using them in multiple would be impractical (only class 700s and 345s fit that category I think).
For those in the know, would the third rail electrification between Portsmouth and Redbridge be able to accomodate a hypothetical dual voltage bi-mode on the route? That would be ideal for any new stock.
I've no idea if the third rail has spare power capacity, but I absolutely agree that any new stock should have 3rd rail and OHLE current collection either built in or an easy retro-fit once wires are extended down Filton Bank into Temple Meads.
To those suggesting end doors: you must not have been on any moderately well loaded 158 passing though Southampton, Bath or Bristol! The Pompey-Cardiff routinely loses a few minutes here and there, and it hasn't a hope of making up any time at stops with end door trains, quite the opposite in fact. Not worth the fairly marginal increase in comfort for longer distance passengers.
The 158s were mainly only 3-car (meaning fewer doors overall and standees clogging up the doorways due to insufficient seating capacity) and have clunky old door mechanisms. A new 5-car 444-like unit would be much better, still not as quick as a Turbo I grant you but allow for it in the timetable so you don't run late as a result and it's no problem. As a long-distance passenger, I would much rather add five minutes to a three hour journey to gain some more legroom than sit for 3 hours with 'my knees behind my ears'.
I would echo other posters suggesting that new stock should be gangwayed, able to take advantage of the (hopefully ever expanding) electrification at each end, and operate at least 5 carriages for the majority of the day.
Yep, gangwayed, able to run in electric mode and 5-car formations. At least we can agree on that much.
