Edinburgh North-South tramline: 'Single track running over Dean Bridge would remove need for strengthening'
Councillors are being urged to avoid potential major costs of strengthening Edinburgh's historic Dean Bridge for a new tramline by adopting single track operation over the A-listed structure.
Having to take trams across Dean Bridge is regarded as a key obstacle to choosing the on-road Orchard Brae route for the proposed new North-South tramline between Granton and the Royal Infirmary rather than the off-road Roseburn Path route, which has met with strong opposition from greenspace campaigners.
Engineers who assessed the bridge “strongly” recommended the council should choose a different route.
But councillors have been sent a letter, from "an apolitical Edinburgh resident", arguing that single track running - already proposed for part of the Roseburn Path route to reduce the loss of greenspace - would reduce the weight the bridge had to carry to below the weight of current traffic.
The letter said: “Calculations show each of the four arch spans of the Dean bridge takes a typical traffic mix of 64 tonnes today, and a peak load of 100 tonnes. As each span needs to support only part of a tram due to its length, a span must support a maximum of 60 tonnes of tram weight.
“As this is less than the 64 tonnes continuous road traffic weight the bridge already supports day-in day-out, year-in year-out, no additional strengthening is needed for a single running tram.”
The letter said the Roseburn Path option involved trams running on single track for one mile. “It's even easier to do this on the Dean bridge, at just one-tenth the distance (136 metres). And easily achieved with traffic lights, an arriving tram would have sole access to Dean Bridge.
“And by placing the track in the centre of the carriageway the bridge provides fully symmetrical support, away from the edges where existing heavy road traffic stress the bridge today.”
The letter argued the inclusion of single track running in the Roseburn Path option but not for Dean Bridge meant the choice presented in the recent consultation was biased against the Orchard Brae option.
“Just 150 metres of single track running removes all the strengthening costs from the Dean bridge option. And so puts both Dean Bridge and Roseburn path options back on an equivalent cost footing.”
Transport convener Stephen Jenkinson rejected the suggestion made in the letter.
He said: “Single track running has a marked effect on journey times, and in the case of the Dean Bridge, would significantly impact the already slower predicted service for the Orchard Brae line.
“Journey times are fundamental considerations for any business case and as such single track is only used in exceptional circumstances. To compare this to the Roseburn Path, where single-track was specifically suggested to mitigate impacts on active travel and ecology, is fundamentally misleading.
“The notion that single track would also somehow mean less work being needed to strengthen the Dean Bridge is false. Structural engineering assessments are far more complex than simply gauging vehicle weight.
“Independent experts spent months preparing the detailed reports which accompanied our consultation, and these baseless and anonymous assertions add little to the debate.
“The tram expansion has hugely positive transformational potential for our city. My focus is on making sure we are prepared for the challenges of the future, and I look forward to seeing the business case debated next year.”