Peak ScotRail fares have been scrapped on routes across Scotland as part of a six-month trial.
The pilot funded by the Scottish government aims to boost rail use and will see the cost of travelling between Edinburgh and Glasgow almost halve.
The scheme began on Monday and is set to run until the end of March 2024.
Commuters are now able to travel all day on off-peak fares, which ScotRail said will see "massive savings" across the country.
The fare from Inverness to Elgin has fallen from £22 to £14.40 and the ticket price from Glasgow to Stirling has dropped from £16.10 to £9.60.
ScotRail explained there are some routes where no off-peak fare exists as the price is the same at any time of the day. Customers in those areas will not see a change in their ticket prices.
The rail operator expects some trains will be busier and will be monitoring services daily.
There will be additional carriages on services between Edinburgh and Glasgow via Falkirk High, with every service operating with seven or eight carriages.
Extra carriages will also be added to services through Glasgow Queen Street Low Level and Glasgow Central Low Level (linking Lanarkshire to Dunbartonshire).
Alex Hynes, Scotland's Railway managing director, said: "We want to encourage more people across the country to choose rail travel instead of using the car.
"Everyone at ScotRail is working hard to make sure that this six-month trial will be a success and we will be monitoring our services and stations daily to see where we have any significant increases in customer journeys.
"We know that cost and simplicity are critical factors for people when they choose how to travel, and we are looking forward to delivering this fantastic fare reduction for our customers."
The project is said to be the first of its kind in the UK rail industry and aims to support the Scottish government's ambition to achieve net zero.
ScotRail said the pandemic radically changed the commuter market, with a "significant drop" in the number of passengers travelling during traditional peak times.
That market is only 70% of what it was before COVID.
First Minister Humza Yousaf previously said the Scottish government had budgeted "around £15m" for the six-month trial.
The pilot's impact and long-term sustainability will then be considered ahead of any potential renewal after March next year.
The Scottish Greens, who back the initiative, said it would encourage people to switch from cars.
MSP Mark Ruskell, the party's transport spokesperson, said: "This is exactly the kind of bold, ground-breaking move that Scotland needs."
Kevin Lindsay, Scottish organiser for train drivers' union Aslef, added: "To encourage people on to trains and away from their cars we have to make train travel much more affordable and this is an excellent first step in the right direction."