A start on tram-trains running between Sheffield and Rotherham has been delayed.
Problems with design work to allow the Supertram-style vehicles to operate on train tracks means the launch date of spring 2016 for the £60million project will not be met.
A revised timetable is due to be announced before the end of this year.
The delay follows another late arrival between Sheffield and Rotherham - a rapid bus service under Tinsley Viaduct.
Government-financed tram-trains are planned to operate between Rotherham Parkgate and Sheffield city centre by linking the existing tramway and the national rail network.
The concept is to be piloted for two years to assess the technical issues and with a view to tram-trains running in other parts of the country.
A joint statement from South Yorkshire Passenger Transport Executive and Network Rail said there were “delays in the design of the works required to adapt the heavy rail network to accommodate the tram-train service.
“Network Rail, who are responsible for these works, are presently undertaking a detailed assessment of their delivery programme, and the project partners are committed to working with Network Rail to mitigate the delay as far as possible. Some of the delay arises from the project’s relationship with other heavy rail investment programmes in order to maximise efficiencies, such as signalling control.
“Good progress is being made on other aspects of the project, with vehicles being manufactured and works to the tramway and depot to accommodate tram-trains well underway.”
But council cabinet member Leigh Bramall said: “The delays to tram-train are a real concern, as it has already been delayed and it’s concerning that the Government are continuing to drag their feet on this.
“None of the delay is down to local partners, it is the Government and National Rail where the problem lies.”
Coun Bramall added: “The scheme is also a big part of our efforts to agree a Sheffield-Rotherham economy and so it’s crucial that Network Rail take this seriously and do everything possible to deliver the scheme as soon as possible.”