SNP ministers have backed plans to create a Glasgow metro system that could better link 1.5 million people living in the west of Scotland.
The ambitious project could see tram lines opened in the city for the first time since the 1960s alongside major upgrades to the existing rail network across eight local authority areas.
Faster and more frequent trains linking the city centre and suburban areas would be created under a plan to introduce "heavy metro" services.
Transport minister Michael Matheson said the multi-billion pound scheme would link people "from Clydebank to Cambuslang".
But the Scottish Government was today accused of failing to provide details on when work will start and how much it will cost.
The metro is one of 45 core projects across the country which have been recommended for investment over the next 20 years.
Matheson told MSPs the second Strategic Transport Projects Review (STPR2) marked a "repositioning" of the government's priorities with an emphasis on plans that will deliver environmental benefits.
"The Clyde metro project represents a multi-billion pound investment which, when completed, could better connect over 1.5 million people - from Clydebank to Cambuslang, and from Easterhouse to East Kilbride," he said.
"For many people in our city regions, having better access to affordable and reliable public transport has the potential to significantly reduce inequalities and enhance opportunities for many who live and work in some of our most deprived areas.
"This project will make a substantial contribution to tackling climate change by reducing car-based trips and associated emissions."
But Matheson could not provide a firm start date for the project when asked.
Labour MSP Pauline McNeil said: "Glasgow city region has not had a serious transport project under the SNP administration since 2007 and we desperately need one.
"The proposal for the Clyde metro has no timescale, no serious funding allocated, and appears more of a concept in a document than a real transport project.
"I ask the transport secretary, when is this project likely to start - when will it be a reality?"
Matheson replied: "The Clyde metro programme will cover about 15 kilometers from the city centre.
"It is built on work already carried out by the Connectivity Commission - we will now look to put in place a programme of work.
"That involves us working with all the local authorities involved - not just Glasgow - and other partners in how we take this forward over a number of years."
The SNP minister added he could not give a specific timescale other than it would happen within the next two decades covered by the transport review.
Greater Glasgow has the largest suburban rail network outside of London but council leaders have argued that large parts of the region are still badly served by public transport.
A previous study recommended a light rail line should be built along the south of the Clyde to link Glasgow Airport, the Queen Elizabeth University Hospital, and the city centre.
Glasgow City Council leader Susan Aitken claimed the finished metro system could have "a bigger footprint even than Manchester’s Metrolink, the closest UK equivalent".
In a document published by Transport Scotland, the metro is defined as "one of, or a combination of, bus rapid transit, tram, light rail and metro rail".
It adds: "These options would complement the service provided by traditional railways and may include the conversion from existing railways to tram or heavy metro."
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