Services in England are being cut from 87% of normal levels to 72%, industry body the Rail Delivery Group said.
It said the number of trains would reflect the drop in passengers, and provide better value for money for taxpayers who are subsidising services.
Peak services will be prioritised to help key workers, it added.
While some timetables have already changed, others will be altered in the next few weeks.
Since the early days of the pandemic, the government has spent billions of pounds covering the fall in ticket revenues for rail companies, owing to low passenger numbers.
Cutting some services will save public money, the government said.
Rail minister Chris Heaton-Harris said: "It is critical that our railways continue to deliver reliable services for key workers and people who cannot reasonably work from home, and that they respond quickly to changes in demand."
Rail usage has slumped, with passenger journeys falling more than 90% to 35 million journeys for the three-month period to June, according to the Office of Rail and Road.
The figures recovered a little to 134 million for the three months to September - the latest published.
"Train timetables must still meet the needs of those who have to travel, said Transport Focus chief executive Anthony Smith.
"Many key workers rely on the first and last services of the day so it's important that these are maintained. Providing enough capacity for those who are travelling to properly social distance remains vital."
Although timetables were restored when restrictions were eased over the summer, rail franchising has since been scrapped and replaced with a model which means the taxpayer is currently liable for the losses on the railways.
In September, the bill had run to more than £3.5bn - and the Department for Transport has said "significant" support is still needed.
source: BBC News
People needing to travel by rail during lockdown are being urged to double-check train times, as services are being reduced.