Less than a year after it opened, another attempt is being proposed to extend the Elizabeth line, this time via a 30-mile loop through parts of Essex and Kent. The proposal, from Thurrock Council, is included in a wider proposal to improve local transport links and could see the two eastern spurs of the Elizabeth line joined in a loop that would run under the Thames.

The suggested proposal is for the Elizabeth line to take over the existing London Overground line from Romford to Upminster. Then it would take over the existing single-track railway that runs between Upminster and Grays, onto Tilbury.

Then it would pass under the Thames to Gravesend, where it would link in with the already proposed extension between Abbey Wood and Northfleet.

There are however some pretty substantial engineering problems to overcome.

The Elizabeth line trains arrive at Romford station on the north side of the station, but the railway to Upminister is on the south side, so a large flyover would be needed to link the two sides up.

There is enough space to double-track the railway to Upminster, but the single platform Emerson Park station is tiny and would need a considerable rebuild.

At Upminster, the same problem as at Romford exists, in that the Elizabeth line trains would be arriving on the north side of Upminster station and would need a flyover to get to the south side for to run over the C2C line down to Grays.

The two existing stations at Chafford Hundred and Ockendon will need rebuilding, and the plans include a new station at South Ockendon.

Once the extension reaches Tilbury, the intention is that it would dive into a tunnel under the Thames to Gravesend. The difficulty is how to link the tunnel with the railway at Gravesend as the station is in the town centre. The most likely option would be to join the railways to the east of the town where there’s empty land suitable for the tunnel portal.

The most likely option to achieve that though would require the railway to head out of Tilbury to the east and then loop down to the river on the eastern side of the former Tilbury Power Stations.

None of these engineering issues are in the report by Thurrock Council, which is admittedly a very high-level aspiration at this stage, but even at this level of detail, the problems are easy to have identified prior to publication.

If the flyovers weren’t needed, an Elizabeth line extension to Thurrock would be expensive, but affordable if funded by substantial housing along the C2C spur between Grays and Upminster. The need for the flyovers makes it almost certainly unaffordable though.

There’s also the often overlooked issue that alongside the railway track and station upgrades, you need a load more trains, and they’re not cheap. At around £15 million each, you’d likely need an additional 20 trains for the extension, plus the depot so somewhere in the region of £350 million on top of the railway infrastructure upgrades.

That would usually come out in more detailed research into the costs. Thurrock Council will however struggle to pay for the necessary research into the proposal to flesh out the details, having recently filed a Section 114 notice barring any new expenditure after it dropped into a £500 million deficit due to failed investments.

It’s not that there’s a lack of need for transport upgrades, as the railways in/out of London are congested and there’s a lack of capacity at Fenchurch Street station to cope with much more. The area is however unusually lucky in having the north-south line between Grays and Upminster, which needs double tracking, or bypass loops added to increase capacity.

The perennial question is about paying for the upgrade.

Just the Abbey Wood to Northfleet section through Kent has been previously estimated to cost around £3 billion — so doubling it through Essex as well, would likely double the cost again.

A large chunk of the current Elizabeth line was funded by property developments, but the area around this extension is largely already developed with suburban housing, and only about 20 percent passes through land that could be developed if Green Belt objections can be overcome.

The council’s report (pdf) does look at how local transport upgrades could be delivered to make local journeys better, and it would likely be substantially easier to upgrade the existing C2C line into London, than to build a giant 30-mile loop extension of the Elizabeth line.