The £12 million project has been undertaken as part of an alliance announced yesterday between ScotRail and Network Rail which is expected to bring down costs and lead to a more integrated railway.
ScotRail is the latest UK train company to form a regional alliance, after recommendations made by Sir Roy McNulty's value for money study, which criticised the fragmentation left in the industry following privatisation nearly two decades ago.
The cost of the Paisley project had been "significantly" reduced as a result and would be completed in less time, ScotRail said yesterday.
This is partially because the train company has waived its usual compensation claims for the disruption it would suffer as Network Rail undertook the improvement work.
Susan Anderson, Network Rail's route commercial manager, said: "This is the first tangible benefit for rail customers to be delivered through this alliance with ScotRail. We are delighted to be able to achieve this much more quickly than under traditional arrangements."
Steve Montgomery, managing director of ScotRail, added: "This work is set to deliver substantial benefits for customers while driving down costs. The ground-breaking alliance is a leaner, more efficient way of working and sets the standard for future rail projects in Scotland."