An urgent investigation into delayed engineering works which caused serious disruption for commuters in the west of Scotland has been announced.
Mary Dickson said First ScotRail was in the dark over a finish date
Services from Inverclyde, Ayrshire and Renfrewshire were suspended or reduced due to £9m signalling work at Shields Junction, south of Glasgow.
Network Rail said the line would close again this weekend.
The Office of Rail Regulation announced an investigation into the work, which was due to be completed on Sunday.
Chairman Chris Bolt said: "Passengers were badly let down by Network Rail's handling of its Christmas and New Year engineering programme.
"Our investigation into Network Rail's planning and management of these projects will be extensive and thorough."
Train operator First ScotRail said Network Rail's handling of the project was "unacceptable".
First ScotRail's managing director Mary Dickson said: "We have reluctantly agreed to the closures on the clear understanding that we expect a full resumption of all services on Monday."
Ms Dickson called on Network Rail to order a full inquiry.
Services have been affected since the festive period.
A spokesman for Network Rail said engineers had made "significant progress" and a full service via Paisley Gilmour Street was reinstated on Thursday afternoon.
However, no trains will run between Glasgow and Paisley this weekend.
On Saturday and Sunday all Inverclyde and Ayrshire services will terminate at Gilmour Street, where replacement buses will operate.
Network Rail said its engineers had faced unprecedented difficulties which only emerged while the work was under way.
David Simpson, route director for Scotland, said he did not expect services to be completely restored until early next week.
He added: "While significant improvements to services have been made, further work will still be required this weekend to allow us to complete the project and restore full services to the Paisley Canal line early next week.
"We understand the anger and frustration which this engineering overrun has caused passengers and our railway industry partners and we would like to reiterate our apologies for the continued delay."
Network Rail said 35 major improvement schemes were carried out during the festive season, of which 33 completed "successfully and on time".
The Office of Rail Regulation said it was extending its investigation into two Network Rail over-runs in England to include the work at Shields Junction.
A report is expected in February.