Virgin Trains says it has permanently removed all peak-hour restrictions to trains that travel on Friday afternoons from London Euston station.
It said the decision was made to ease overcrowding on Friday evening off-peak trains to destinations such as Birmingham, Manchester and Liverpool.
Passengers on off-peak trains are often forced to stand for hours in order to avoid paying much higher peak fares.
Transport watchdog Transport Focus said it welcomed the move.
Customers can now choose to travel any time in the afternoon, and they will be charged off-peak ticket prices.
"Virgin deserves praise for testing a real practice improvement in the service they offer hard-pressed passengers, and for following through to make a permanent change when demand clearly speaks for itself," Transport Focus' chief executive Anthony Smith told the BBC.
"Virgin's experience suggests some clear lessons for other operators too - we do hope others do not wait long to follow suit."
Previously, a return ticket from London to Manchester that was bought on the day and valid for peak travel would cost £212.45. In contrast, an off-peak return fare costs £86.90, without having to book in advance, giving a saving of £125.55.
However, customers can still get cheaper prices on train tickets should they choose to make an advanced booking.
Virgin says that the off-peak return fares on Friday afternoons offer savings on the following destinations:
- London to Birmingham - £60.50
- London to Glasgow - £111.25
- London to Manchester - £125.55
- London to Preston - £129.95
- London to Liverpool - £115.55
- London to Stoke-on-Trent - £105.20
- London to Stafford - £75.20
- London to Coventry - £53.50
- London to Chester - £94.55
Virgin initially conducted a 13-week-long trial where peak fares were scrapped on Friday afternoons, which saw congestion reduce by over 50% on many routes on trains leaving London Euston station after 7pm.
Tom Burridge, transport correspondent
A cut in rail fares - albeit for people travelling on Friday afternoons on Virgin - is a rare, but welcome concept for passengers. Flexible working patterns of today demand more flexible prices.
Despite the considerable saving it offers to passengers travelling at those times, the train operator claims the change will have no impact on its revenue. It says that during the 13-week trial it saw a rise in the number of passengers catching services earlier in the day.
However the latest figures from the rail regulator suggest the company needs to improve its punctuality. In the first quarter of 2018, 85% of its trains arrived on time.
No more peak fares on Friday evenings is good news for passengers but politically-speaking this is a sensitive moment for the rail industry as a whole and many will feel that £86.90 (the off-peak fare) is a fairer price for travelling 160 miles (London to Manchester) on a train.
"Rather than everyone waiting for the first off-peak train on Friday evening, people can now travel whenever's convenient for them," said Virgin Train's commercial director Sarah Copley.
"This change means we've been able to reduce congestion, whilst helping people save money and make an earlier start to their weekends."
The Rail Delivery Group, which represents the railway, told the BBC that it ran a national consultation in the summer in order to improve rail fare regulations.
"This is welcome news for Virgin Trains customers. As an industry we want to build on initiatives like this and go even further, by getting rail fares regulations brought up to date so we can deliver an easier, more flexible system that fits with how people work and travel today," said the Rail Delivery Group's managing director of customer service Jacqueline Starr.