An update on the project by the Scottish Government agency said a “significant impact” was likely on train services on the main Paisley-Glasgow line – one of Scotland’s busiest routes.
Tram-trains that could run on rail lines to the city centre are seen by ministers as a possible alternative to the Glasgow Airport Rail Link, which they cancelled in 2009 to save money.
A report last year showed the scheme could cost £92 million - less then half as much as a rail link- but be able to carry as many passengers.
Tram-trains would run on a dedicated line from the airport terminal then continue to Glasgow on the rail line from Paisley.
The new report said there were “constraints on line capacity” on the approach to Glasgow Central Station for the planned four tram-trains an hour.
It also said space at Central was “very limited”, despite two extra platforms being built for airport trains, which are now used by other services.
However, the report said this could be solved by removing some train services from the station to create space for tram-trains, or running tram-trains on streets into the city centre. Journey times would last 20 minutes 30 seconds – only 90 seconds faster than airport buses.
The report also said a long-delayed tram-train trial in Sheffield, due to start next year, would have to be completed before “absolutely definitive” proof that tram-trains could run on rail lines was available. But, it stated: “The indications from the pilot are positive so far.”
A key consideration is likely to be the safety risk of tram-trains running alongside faster, heavier trains.
The project would be funded under a £1.1 billion “City Deal” for investment in Glasgow, agreed last year between the UK and Scottish governments.
However, the report said it could still be ten years away.
A Scottish Government spokesman said: “The study makes clear there are significant challenges to the delivery of a tram-train link, which would raise financial, technical and operational issues.
“As well as the cost to taxpayers, we have particular concerns at the disruption the scheme would have on rail passengers and existing services – including the risk that journey times would increase and the frequency of other services be reduced.”
Business groups, which had expressed their “profound disagreement” at the Glasgow Airport Rail Link being scrapped, reaffirmed their backing for an alternative link to the airport and urged more work to overcome the problems.
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