Skip to content
Case study

How Transport for London reduced the Bakerloo line extension’s whole-life carbon footprint

05 April 2022

Whole-life carbon modelling identified a number of emissions ‘hotspots’ in the proposed extension – redesigning these allowed TfL to cut the project’s footprint by almost 30%.

How Transport for London reduced the Bakerloo line extension’s whole-life carbon footprint
The proposed Bakerloo extension would extend the line from its current terminus at Elephant and Castle to Lewisham.

Trains are a relatively low-carbon mode of transport. Figures from the Department for Business, Energy and Industrial Strategy (BEIS) show that London Underground produces significantly lower operational carbon than other transport modes including National Rail, buses, cars and aircraft.

A London Underground extension would therefore seem to be an obvious choice when trying to decarbonise the capital’s transport network and increase its capacity.

However, these figures don’t allow for the capital carbon involved when building such a scheme (capital carbon being emissions associated with the creation of an asset). So what is the whole-life carbon footprint of an Underground line, and can we do better?

Minimising the Bakerloo line extension’s carbon footprint

A Bakerloo line extension from Elephant and Castle to Lewisham would create significant additional rail capacity in south-east London, enabling a major mode shift from road to rail and unlocking growth in homes and jobs.

A feasibility design for the extension used feedback from the 2017 public consultation to inform key decisions including station locations, tunnel route, and where the tunnel should connect to the existing line.

Given that whole-life carbon measurement is becoming a greater consideration in infrastructure investment and that other high-profile projects are being challenged on their carbon credentials, Transport for London (TfL) conducted an internal review of the project to minimise its potential footprint.

Proposed Bakerloo Line extension
Proposed Bakerloo Line extension

The UK Government’s Infrastructure Carbon Review places significant responsibility on clients to develop low-carbon solutions. It has also provided a mantra of “reducing carbon reduces cost” and a hierarchy for achieving this. The objectives of this work by TfL were to reduce the whole-life carbon footprint of the scheme and quantify the savings.

Building the whole-life carbon model

A whole-life carbon model was constructed in Microsoft Excel using an evolution of previous carbon models. This combined a range of data sources for the design (such as cost estimates, power estimates, asset life and so on) and carbon factors for materials, energy and transport (from the Infrastructure Carbon and Energy database and BEIS forecasts).

The model assessed the design across PAS 2080 modules A, B and C in as much detail as possible given the level of detail in the design information. The assessment period was taken as 60 years to match transport analysis guidance for the development of the business case. The carbon model took approximately two days to produce.

An assessment of the base case model identified a number of carbon ‘hotspots’, specifically: running tunnels, trains, depots and the proposed station at Old Kent Road. These areas were the primary focus of the design work to achieve maximum carbon savings with minimum effort.

Design revisions

The most significant design changes arose from challenging the basis of design from the previous stage. In understanding what had driven the highest-carbon elements, the current design efforts could focus on agreeing key issues, considering different scheme options in light of their carbon footprint and providing only what was truly required.

The base case was designed to operate at up to 36 trains per hour; however, such a level of service adds significant cost and carbon through more trains, power and infrastructure.

Reducing the frequency to 27 trains per hour, while at the same time not precluding a 36-trains-per-hour service in future once needed, could allow the number of new trains to be reduced by six, power consumption to be cut by approximately 40% and the depot to be removed entirely (along with the associated 3km of running tunnel).

The Old Kent Road station was proposed to be a 300m-long box containing both the station and a crossover. The size of this station box was driving significant carbon through the amount of concrete needed to construct it and the associated cost. The crossover was removed with only minimal impact on the operation of the railway, and the station was redesigned to reduce its length to 130m, using lessons learnt from the Northern line extension.

The revised design was also applied to Burgess Park and New Cross Gate stations, reducing their volumes by about 20%.

The Lewisham Way intervention and ventilation shaft had been designed with all of the ventilation equipment located inside the shaft. This would result in an extremely large shaft that needed to be constructed using diaphragm walls.

By relocating the equipment to a headhouse, the shaft’s diameter could be significantly reduced and a caisson method could be adopted using precast units. Overall, the excavated volume was reduced by 75%.

A range of smaller changes were also made to the extension, along with a significant number made to the planned upgrade of the existing Bakerloo line.


The revised design was remodeled and compared with the baseline design. Overall, the whole-life carbon estimate was reduced by 28%, from 800,000 tCO2e to 570,000 tCO2e. The most substantial carbon reductions were seen where a ‘build nothing’ approach could be adopted, with large savings where ‘build less’ could be used.

The split between capital and operational carbon was approximately 65%/35%, meaning twice as much carbon would be emitted during construction compared with operation. The carbon model factored in grid decarbonisation so the long-term emissions from energy reduced over time. For rail to become an even more sustainable mode of transport, it needs to focus on reducing the capital carbon of the infrastructure.

Graph showing Bakerloo Line Extension Carbon Footprint
Graph showing Bakerloo Line Extension Carbon Footprint

Notably, the proposed Burgess Park and New Cross Gate stations saw relatively small changes in carbon despite the design revisions, while Lewisham station saw an increase.

This was caused by an increase in the level of design detail for the station boxes, meaning more of the station could be quantified. Hence, the genuine carbon saved from reducing the size of the box was masked by improved model accuracy.

While commercial estimating benefits from accumulated knowledge of historic project costs to benchmark against, enabling appropriate levels of risk to be applied at each stage to reach reasonably good estimates, carbon modelling is still relatively new so the amount of risk to apply at each design stage is unclear.

Estimating the true carbon footprint was not essential at this time because this piece of work was attempting to minimise the total footprint. Understanding carbon risk through the design stages will become essential once projects begin trying to demonstrate that they will achieve net carbon neutral.

The estimated whole-life carbon can be converted to a financial value for use in policy appraisal using the marginal abatement costs set out by BEIS. This gives an approximate abatement cost of 3% for the initial construction. Considering the scale of the project, this cost is unlikely to be a primary decision driver.

The provisional capital cost savings from the design changes were much more significant – approximately 40%. This reflects the Infrastructure Carbon Review mantra and is likely to be a primary decision driver.

The road to net zero

Modelling to date has considered only the gross carbon associated with building and operating the railway. Future design stages will look to make the design even more efficient and introduce new or alternative technologies to minimise the gross carbon footprint of the Bakerloo line extension as much as is practical – still, it’s unlikely that the project could achieve zero gross carbon.

Net zero carbon may, however, be possible by considering elements outside of the operating railway. The purpose of the Bakerloo extension is to bring about improvements to south-east London. Where these improvements reduce carbon emissions elsewhere then some of this could offset the extension’s carbon; these benefits can be accounted for in PAS 2080 module D.

One of the main objectives of the project, and the area with the biggest net-carbon reduction potential, is to move passengers towards more sustainable modes of transport. The amount of mode shift from road to rail can be estimated and the carbon saving quantified to offset the project’s carbon.

Noting that road vehicle emissions will fall with growing electrification, the sooner this mode shift happens, the greater the carbon saving will be. Even allowing for this, however, rail is expected always to have lower operating emissions owing to inherent efficiencies from features including low rolling resistance and no losses from battery charging.

The Bakerloo line extension would also enable the construction of new, more efficient homes in a sustainable location with excellent access to public transport and so less reliant on individual car ownership.

Opportunities therefore exist for symbiotic relationships between the new homes and the extension – for example, using waste heat from the stations to reduce heating demand. These new properties should be far more carbon efficient than existing homes, allowing further offsetting of the Bakerloo extension footprint.

One of the next stages of work will look at the wider effects of an extension from a carbon perspective. Some of these benefits will be difficult to quantify. If the wider benefits of the scheme do not offset the estimated whole-life carbon, then other means of achieving net zero may need to be employed.

Future potential

This work estimated the whole-life carbon footprint of the proposed Bakerloo line extension, identifying the most carbon-intensive elements of the scheme in the process. Following the Infrastructure Carbon Review hierarchy, these elements were targeted with additional design efforts, meaning a 28% reduction in whole-life carbon could be achieved.

While this is a good first step, subsequent design stages will have to continue reducing the footprint through better design and consider the wider effects of the project or offsetting to achieve net zero carbon. This approach, of considering the footprint at the earliest stages to enable significant carbon and cost reductions, could be adopted by any project.

The Bakerloo extension’s carbon model has formed the foundation for a new tool that could be applied across a much wider portfolio of TfL projects and beyond.

Be an ICE Carbon Champion

ICE’s Carbon Champions initiative aims to celebrate individuals and their teams who are committed to achieving net zero. Applicants are invited to submit their examples of carbon reduction in practice, giving details of their projects’ carbon savings.

Apply to be an ICE Carbon Champion

Name of Project: Bakerloo Line Upgrade and Extension

ICE Carbon Champions involved in this project:
  • Matthew Lees, Transport for London
  • Jane Wright, Transport for London
  • Matthew Lees CEng MICE, senior project engineer at Transport for London