The Scottish Government is being urged to scrap peak rail fairs and offer free travel to people over 60 and under 24 ahead of the announcement of the budget.
Deputy First Minister John Swinney will outline tax and spending plans for the next financial year next week, against the backdrop of rising inflation and prices, as well as public sector pay negotiations.
In a letter to Mr Swinney and First Minister Nicola Sturgeon, the train drivers union Aslef said reforming fares on Scotland's railways would contribute to meeting net-zero targets and help Scots during the cost of living crisis.
Currently, a peak return fare on a ScotRail train between Glasgow and Edinburgh is £27.60, while an off-peak return is just £14.20 between the same two stations.
This week, the Climate Change Committee (CCC), said Scotland has missed seven of its 11 net-zero targets to date and there was "no clear plan of delivery" that would see emissions cut by 75% by 2030.
In his letter, Aslef Scottish organiser Kevin Lindsay said: "With this in mind, we would urge you in this budget to help increase rail travel by abolishing peak fares, which we see as another tax on workers, and also to make rail travel free for the under-24s and over-60s.
"By directly intervening in this way, your Government can help the travelling public break from the bad habit of unhealthy and environmentally damaging road travel and get into the good habit of environmentally sustainable rail travel.
"Moreover, such a policy will hugely help people who are struggling in the face of a very harmful cost-of-living crisis.
"If the Scottish Government is serious about meeting its climate change targets, these policies are the very least the Government should be doing.
"We urge you to take heed of our call, set against the context of the very serious assessment this week from the CCC, and end peak fares and introduce free travel for under-24s and over-60s."
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