Rail travellers are being warned of major disruption as an £85m refurbishment of one of Scotland's busiest stations begins this weekend.

Network Rail yesterday said the first part of the operation, which will include work on signals, the creation of a new engineering access point from Commerce Street and the refurbishment of a signalling gantry, will affect services to and from Glasgow Central High Level Station on Saturday and Sunday.

Trains will be replaced by buses for all or part of their routes.

A Network Rail spokesman said the public will see little visual difference after the work but improvements will be noticed in train reliability and punctuality.

The upgrade is part of a wider £2.4bn expansion of the railways across the UK.

The programme will include the lengthening of hundreds of platforms, raising speed limits on some lines, new tracks and major resignalling schemes.

It will also include around two-thirds of a £300m new line between Airdrie and Bathgate, which is planned to be operational by 2010.

David Simpson, of Network Rail, said: "The upgrades are essential to improve the reliability and performance of all trains arriving and departing from Glasgow Central.

"They will provide a welcome modernisation of the station infrastructure and also improve the long-term performance of rail travel in Scotland. We have carefully planned this work and our engineers will work round the clock to carry out these improvements."

Steve Montgomery, First ScotRail's operations and safety director, said: "These works will involve a high number of replacement buses and it will be a major exercise to manage passengers to and from their services.

"We are confident the planning on travel alterations will keep inconvenience to a minimum. Clearly, some journeys will take longer than normal and passengers should allow more time for travel."

Virgin and GNER services will also be affected by the engineering work at Glasgow Central but Low Level (Argyle Line) trains are unchanged.

Between Friday and Tuesday, engineering works will also affect services to and from Queen Street Low Level station. In addition, between Saturday and Monday services between Edinburgh and Perth, Dundee and Inverness will be diverted because of work at Inverkeithing, with replacement buses running between Inverkeithing and Perth and Dundee.

In Edinburgh, the on-going redevelopment of Waverley Station, involving the building of two new platforms, longer platforms for commuter trains and new escalators and lifts, is on track for completion by the end of the year. It will see four more trains every hour into the station.

Yesterday's announcement followed an unhappy period for Network Rail. In February, the company accepted the initial findings of a preliminary rail accident investigation that said the Cumbria derailment, in which a Glasgow woman died, was caused by faulty points.

Then the Office of Rail Regulation expressed concern that Network Rail was underspending. Finally, last week it was fined £4m for breaches of the Health and Safety at Work Act by its predecessor Railtrack in connection with the 1999 Paddington rail disaster which claimed 31 lives.