The joke goes that SPT stands for "Studying public transport - because that is just about all it does".

The once powerful Glasgow-based agency, properly called Strathclyde Partnership for Transport, certainly spends a lot of its time - and taxpayers' money - investigating the transit options of the future.

Schemes under review include a bigger, better Subway; river buses; and a new maglev train to Edinburgh. Some, industry insiders admit, are pie in the sky. Others, they say, are vital for the Scottish economy.

Yesterday the agency was finally given what one senior industry source described as its "reality check".

It was stripped of the right to deliver its flagship capital project, the £210m Glasgow Airport Rail Link (Garl). The bottom line, according to one insider, was they were not "an infrastructure provider".

SPT officials - including the agency's chairman, Glasgow Labour councillor Alistair Watson - were yesterday playing down the snub.

"I am not precious about who cuts the ribbons," said Mr Watson yesterday. "SPT can always be seen as the organisation that made the case for the airport rail link when the establishment in Edinburgh did not."

The question is what does SPT do now? It still has its studies to do, and not just into futuristic new schemes.

It has a mundane but strategic role in developing public transport for all 12 of its constituent councils, essentially the old Strathclyde region.

But, jokes aside, the agency has more than studying to do. It still runs the Glasgow Subway, one of the world's oldest.

Here the agency has had success: the number of journeys on the underground loop have jumped nearly a million in the last year, to more than 14 million a year. Talks are under way to extend its opening hours.

SPT is also still responsible for arranging travel to school for tens of thousands of children and for running "uneconomic" bus routes and Glasgow's Buchanan Street Bus Station.

It has also developed park and rides at railway and Subway stations, achieving, in recent years, a remarkable capacity of 98% as demand soars. SPT is also responsible for the ferries across the Clyde, both from Yoker to Renfrew and from Gourock to Kilcreggan and Helensburgh.

The agency also lobbies for transport in the west of Scotland - officials were last night in Aviemore, making the case for their other big capital priority, CrossRail, the much-awaited link between commuter rail networks north and south of the Clyde.

But will it be allowed to build CrossRail? Mr Watson yesterday claimed SPT was more than capable of doing so.

The agency, after all, is currently rebuilding Partick Interchange. The £12m scheme is behind schedule. Work should have finished last month.

SPT may have the same initials as the old Strathclyde Passenger Transport, the body it replaced in 2006 to make way for the new Transport Scotland agency. But the new SPT has little of the clout of the old.

Officials opted for the same abbreviation to save money on signage. The new SPT has none of the old's powers over railways, having been stripped of its right to be a signatory to the ScotRail rail franchise.

Mr Watson spotted the threat to his body shortly after he took up his post five years ago. The last Labour-led Scottish Executive wanted more national transport planning.

The government that has replaced it appears to want to see Transport Scotland deliver projects as well as plan them.