Electric trains are now running on the line between Edinburgh and Glasgow via Shotts.
New class 385 ScotRail trains have replaced ageing diesel trains on the route, which also serves Uddingston, Bellshill, Holytown, Carfin and Cleland.
The operator, Scotrail, said they are 18 per cent more energy efficient, quieter and have no emissions.
They join the already-electrified other routes - the main line via Falkirk High and those via Bathgate, Carstarirs and Cumbernauld.
The £160 million upgrading of the Shotts line to Glasgow Central was completed earlier this month by Network Rail on time and budget.
Platforms have been upgraded or extended at nine stations, step-free access created at West Calder, and Breich and Livingston South overhauled.
The work will increase capacity to cope with expected passenger growth, including from Livingston and West Calder to Edinburgh, and from Cleland and Shotts to Glasgow.
ScotRail said more than 50 of its 70 class 385s were now in service, having been delivered late by manufacturers Hitachi.
They also operate on the Edinburgh-Glasgow lines via Falkirk High and Cumbernauld, between Edinburgh/Glasgow and Dunblane and Alloa, on the Cathcart Circle in Glasgow, Glasgow Central – Newton and Edinburgh to North Berwick/Dunbar.
ScotRail chief operating officer Angus Thom said: “We are delighted to be introducing the first Hitachi class 385 electric train on the Shotts line and we thank customers for their patience during the improvement works.
“The class 385 trains have proven very popular and this is another step forward for ScotRail as we provide customers with a vastly improved service, including more seats and improved journey times.”
Transport Secretary Michael Matheson added: “This £160m investment in the Shotts line will be genuinely transformational, enabling new, longer and faster trains and providing a better travelling experience for passengers all along this route for many years to come.
“As well as improving connectivity between North Lanarkshire and West Lothian, the investment will both stimulate growth in passenger numbers and make it easier for people to connect to employment, education and leisure opportunities in our largest cities.”
Iain McFarlane, Network Rail route delivery director for infrastructure projects, said: “We are delighted to have completed this project on time and on budget as it will help to deliver additional capacity and journey time improvements to support both passenger and freight growth.
“Electrification is transforming travel across the central belt of Scotland – increasing the number of seats, reducing journey times and cutting emissions by introducing more modern and greener trains to the route.
“We appreciate that this work has at times been inconvenient for those travelling on the route. However, we are confident that the longer-term benefits that the work enables will prove to be worthwhile.”