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Only four UK cities currently have underground transport systems. Could Cambridge become the fifth?
The university city has been front and centre of the government's plans for growth, with 150,000 new homes earmarked by 2050 and a development corporation just given the green light.
As chancellor, Rachel Reeves wanted to create "Europe's Silicon Valley" by connecting Cambridge with London, in the hope of adding up to £78bn to the UK economy by 2035.
But with its centuries-old streets and buildings, designed long before cars, Cambridge faces the possibility of traffic congestion doubling by 2031.
And a confidential report obtained by the BBC through the Freedom of Information Act claims its streets already cannot accommodate the number of drivers there at peak times.
What is the problem in Cambridge?
New public transport systems, including trams and a light rail network, are being investigated by the Cambridge Growth Company (CGC) – set up by central government – alongside the mayor's office, the Cambridgeshire and Peterborough Combined Authority (CPCA).
Confidential reports drawn up for the CGC from March this year, obtained by the BBC, state: "Under a 'do nothing' scenario, where no significant new transport interventions are introduced, the impacts of planned growth are severe."
It forecasts traffic levels would increase by more than 30% in the next five years and "time spent in congestion is expected to more than double compared to current conditions".
It says a "do minimum" scenario "which assumes incremental improvements such as limited bus priority measures and junction upgrades, offers only marginal gains".
Therefore, it suggests, there is a "compelling case" for mass rapid transit (MRT) in Cambridge, where congestion charge plans were dropped a few years ago.
Could tunnels be a solution?
While no decisions have been made yet, one of the reports looks at "potential MRT tunnelled options".
If this were to go ahead, it would mean Cambridge joining London, Glasgow, Liverpool and Newcastle as cities with underground transport systems.
It is not the first time this has come up, however.
Under former Conservative mayor James Palmer, plans were drawn up for a Cambridge Autonomous Metro (CAM), about 31 miles (50 km) of new busways and nearly four miles (6.4km) of tunnels.
But the proposals were dropped in the face of costs of between £2bn and £4bn after Labour mayor Nik Johnson took charge in 2021.
The new report uses these proposals as part of its analysis, saying: "Tunnelling under Cambridge is shown to be feasible from successful tunnelling projects in similar ground conditions and within tight urban site constraints."
But it cites potential problems as high upfront capital costs, complex logistics and less flexible routing depending on buildings.
Proposals for a tunnel running from Cambridge central station, through the city centre and out to the west, where a number of research centres and laboratories are, have been put forward by independent transport group Cambridge Connect.
Its director Colin Harris told the BBC its tunnel would be 4km (2.5 miles) long – shorter than the CAM proposals – and could cost £300m, plus about £100m per underground station built.
He said "the tunnel itself is fairly straightforward in the Cambridge geology of clay", but that a "lot more detailed engineering feasibility work" was needed.
Asked how realistic getting funding for the project would be, he said: "It's whether you're getting the benefits and the payback for what you've invested... in that sense it is feasible, because Cambridge is one of the areas in the UK which is actually driving the economy and showing growth."
The CGC report said the route would wish to "avoid as many of the heritage buildings as possible" and that it was "better to reduce the number of under-river crossings required".
A spokesperson for the CGC said: "We are taking an evidence-led approach to consider what an MRT solution may be for Greater Cambridge, therefore all options are being considered at this point."
They said the reports "in no way either endorse or suggest a future approach".
"The evidence and assessments will lead us towards a set of preferred options but, at this stage, will not set which modes or technologies should be taken forward or, how they will be delivered."
What about other options?
The county's current Conservative mayor Paul Bristow pledged to build light rail – a system of urban trams – when elected in 2024.
Another report obtained by the BBC looks into repurposing the existing guided busway, which connects Huntingdon, St Ives and Cambridge.
It concluded that while light rail in its place could support the expected growth, "implementing these systems on the current alignment would necessitate prolonged full closure of the busway".
It also found the cost to convert it would likely be near the same as constructing new light rail infrastructure, and in a best-case scenario, take about 11 years to complete.
Two other busways are planned, one between Cambourne and Cambridge and one for the south-east of the city, while Cambridge South railway station opened last month.
In a separate Freedom of Information response from the CPCA, the BBC obtained a longlist of transport "interventions".
Options for MRT in the city centre include the possibility of converting Mill Road, Histon Road, Milton Road, Newmarket Road, and parts of Hills Road and Trumpington Road, to "one-way to enable implementation of MRT lanes".
Another option would be for Histon Road to be one-way outbound and Milton Road one-way inbound "to provide space for MRT lanes with signal priority", while another could see some routes "banned to general traffic".
A CPCA spokesperson said it was working with the CGC to develop a strategic outline business case for a potential MRT.
"No option has been selected or ruled out, and no decisions have been made," they said.
"We would not speculate on individual technical details ahead of that work concluding."
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