In an unprecedented open letter ScotRail and Network Rail said sorry for disruption to services between Glasgow and Ayrshire, Renfrewshire and Inverclyde over the past year.
The letter states: "We are sorry that performance over the past 12 months was below the level you expect from us."
It said the disruption had a number of causes, including severe weather, vandalism, and the impact of major engineering works in connection with the £170million improvements along the lines between Paisley and Glasgow.
It also added these vital works by Network Rail are now complete – and increasing capacity, flexibility and reliability on one of the busiest passenger and freight routes across Scotland.
The letter said they appreciated customers' patience while works were carried out. It added: "We fully understand the inconvenience caused while the works took place and appreciate your patience during disruption."
Steve Montgomery, ScotRail's managing director, and David Simpson, Network Rail's route managing director, Scotland also insisted: "Punctuality and reliability levels have now returned to target, but we remain committed to bringing about further improvement.
"Our priority has been to return our services back to the levels you deserve, and we have worked closely to achieve this, with much better performance in recent months."
The open letter also told of a new fleet of modern, comfortable electric trains now serving the Ayrshire, Inverclyde and Renfrewshire routes, and of investments in improvements to track and signal reliability, tackling vandalism, and increasing resilience to severe weather.
And it revealed new services will be introduced in the December 2012 timetable, with details to be announced in due course.
The letter also said: "Thank you for your patience. We are committed to giving more people than ever before the opportunity to travel by rail."
Business groups and local authority chiefs said they were encouraged by improvements and say rail links are essential for their local economies.
David Anderson, chief executive of South Ayrshire Council, said: "I'm pleased rail punctuality and reliability levels are now much closer to those our residents, workers and visitors expect."
Bob Davidson, chief executive of Renfrewshire Chamber of Commerce, added "We hope this will now allow the businesses and people of Renfrewshire to once again travel around in a timely manner."