The Scottish Government's Transport Scotland agency, which is responsible for such roads, said more than 400 staff and 160 vehicles had been working round the clock to clear snow and ice.
Transport minister Stewart Stevenson said: "Scotland has been in the grip of the worst snow and ice conditions in the early winter since the 1960s.
"Every effort has been made to keep disruption to a minimum. Snow was predicted for today but was significantly heavier than expected and timing during rush hour made conditions worse. Staff are working hard to clear the delays caused today, and this evening train services are beginning to improve."