Hounslow mainline to Heathrow line

Given possible expansions of Heathrow Airport, how feasible would a line from Hounslow (mainline, not Tube) be? I started writing this and came up with more problems the longer I thought about it. Dealing with the problems first, in no particular order:

• New third-rail electrification
• Slow compared to Paddington
• Lack of Waterloo / SWML capacity
• Other priorities for Heathrow, London...
• Lack of platform capacity at Heathrow
• Relatively low SWR stock density
• Less money from trains for the airport

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• I don't know whether the ORR would be happy with new third rail electrification. Going off of the Northern line Battersea extension, underground cases might face less resistance.
• Hounslow to Waterloo is currently ~38 minutes (so likely 45+ minutes from Heathrow). Even factoring in taking longer to change for unfamiliar tourists and with more luggage, Heathrow to Waterloo (or Victoria) is unlikely to take more than an hour via Paddington, or even via Earl's Court.
• I don't know how much capacity there is beyond Hounslow having two fewer tph to Waterloo via Brentford than pre-Covid. 2 tph doesn't seem enough to justify a line of this likely cost, there might not be anywhere else for them to terminate, and anywhere short of Clapham Junction seems like a waste as well.
• Having a third rail link from Heathrow to central London doesn't seem likely to go down well if it's instead of rather than as well as a long list of other projects: Heathrow to Slough or Staines, Crossrail 2 (which would also likely mean more services could run on a Hounslow to Heathrow connection if built), southeast Bakerloo line, rail links to Bristol, Leeds or Glasgow Airport...
• 4 tph at Terminal 4 and 6 tph at Terminal 5 mainline stations, all terminating, doesn't leave tonnes of capacity with current platforms. I know that 4+ tph using the same platform's theoretically possible (e.g. on the Watford DC/Lioness line) but it isn't particularly desirable, especially with the risk of importing delays. As fun as it is to imagine Piccadilly line trains running into Waterloo, I'd be surprised if there's enough capacity or timetable resilience to use the Underground platforms either, especially assuming TfL would prefer to see more frequent Acton services.
• I'm fairly sure that all current SWR stock has fewer seats than 345s or 1972 or 2024 Stock (but not necessarily fewer than Electrostars), so wouldn't make a massive difference in terms of total capacity.
• Heathrow might not see more non-Heathrow Express options (or Hayes & Harlington line; do they get money from Elizabeth Line services?) as a net benefit financially, though I'm sure they'd prefer having a third runway approved and no Heathrow Express at all to two runways and fewer rail competitors.

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After that fairly long list of problems, onto the advantages.

• 6 tph (being optimistic) would take Heathrow services up from 22 tph to 28 tph, which is still a sizeable increase.
• Between Waterloo / Waterloo East and Clapham Junction, most of south London and Kent to Dorset could get to Heathrow with one change; useful if Gatwick doesn't have cheap or convenient flights or is more expensive.
• Hounslow and stations towards Waterloo would have a more frequent service (assuming skipping stations neither saves time nor is desirable due to Heathrow Express competition). This is very much using a sledgehammer to crack a nut.

Don't forget in trying to put more trains between Brentford and Hounslow that the capacity is earmarked for the possible 'West London Orbital' service to Hendon.

I don't see a space in the streetscape for a junction off the Hounslow Loop towards Heathrow. Where are you thinking a tunnel portal would be placed?

JonathanH said:

Where are you thinking a tunnel portal would be placed?

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I wasn't thinking about that and came up with half a dozen problems, plus the ones you've mentioned, before I could think about that aspect any more.

I think all the times it was looked at in the past level crossings where also highlighted as an issue. They would be down for an unacceptable amount of time for the motor-vehiles

I think some of the early proposals of getting a direct train will Reading will have gone because of the opening of Old Oak common station.

The arguments of getting into the SouthWest region are still strong in my option. It will never compete on time with the Elizabeth line. However getting passengers to Clapham Junction will be very positive. Just in the same way someone travelling from Bracknell - Gatwick Airport would change there. East Croydon - Heathrow? I suspect will really depend on time / comfort to if someone would choose to change at Clapham Junction or Farringdon.

I don’t think Heathrow Express is competition anyway. The time you spend on the tube getting to somewhere such as Waterloo, I think you will probably have a better journey on any kind of service taking you towards Waterloo directly, even if it did stop at every blade of grass.

Extend the Brentford branch from Southall the short distance into the Hounslow Loop line (on a viaduct - about a quarter of a mile), which has been mooted in the past, and a service can run to Clapham Junction. This would go some way to solve the problem.

Armchair Bob said:

Extend the Brentford branch from Southall the short distance into the Hounslow Loop line (on a viaduct - about a quarter of a mile), which has been mooted in the past, and a service can run to Clapham Junction. This would go some way to solve the problem.

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It solves some of the issues with my original suggestion, whilst keeping others (e.g. Heathrow platform capacity), and I don't know what the best rolling stock would be (700s already have experience with airport services, but I don't know if there's any which could be spared or could be built as a relatively small order).

Why specifically from Hounslow, rather than Feltham? It’s twice the distance, with no viable worksite, no fast trains from Waterloo. Add on some works to close level crossings. After that, how was the play it seems doable. Is there demand for it? Hounslow is near the tube station with good service. Feltham and Kingston have direct and relatively speedy buses, Richmond and Twickenham do not.

SynthD said:

Why specifically from Hounslow, rather than Feltham? It’s twice the distance, with no viable worksite, no fast trains from Waterloo.

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That's another case of me simply not having thought it through. I don't know off-hand if Richmond's still missing services compared to pre-Covid, which Hounslow is, though whichever capacity issues there are would be the same by Barnes either way.

Any new provision from Heathrow needs to be towards the south-west - to Woking and beyond (Basingstoke and Guildford)

More London-bound is just a waste of a scarce resource.