There will be three fast passenger trains an hour between the cities via the Hope Valley line, compared with two at present, once a long-awaited upgrade has been completed.
There will also be a stopping service every hour, rather than every two hours.
For that to happen, a major overhaul is needed to enable passenger services to overtake slower freight trains on the route.
A 1,100-metre passing loop must be created between Bamford and Hathersage, and a second track added at Dore and Totley station, among other changes.
That project finally got the go-ahead in February 2018, and Network Rail said at the time that, subject to funding, work was due to begin in 2019 and take up to three years.
But neither Network Rail nor the Department for Transport have ever given a more detailed timescale.
A passenger group now says it understands the upgrade should be completed by December 2022 – with the extra trains expected to begin running by May 2023.
Chris Morgan, chairman of the Friends of Dore & Totley Station, said: “We’ve been told they’re aiming to have the improvements completed by December 2022. I would have liked it to have been December 2021 but I’ve been told it won’t happen that soon.
“I’m quietly confident we will have the extra trains timetabled from May 2023 but hopefully having the upgrade completed will make services more reliable before then.”
That timescale has not been publicly confirmed by Network Rail.
It said in a statement: “We are working with the Department for Transport to develop designs for the scheme and are liaising with our supply chain to plan this work. This process, for a project of this size and complexity, is expected to take two years.”
Once the upgrade is complete, the introduction of extra services would be a decision for the Department for Transport, Rail North and the train operators.
Passengers using the Hope Valley line through the Peak District can expect some improvements in the more immediate future.
Northern’s ageing Pacer trains are due to be replaced with new or refurbished trains from this summer, and the operator plans for the last of its Pacers to be removed from service by the end of the year.
Transpennine Express, meanwhile, is introducing longer trains on the Sheffield-Manchester route to ease overcrowding.
The operator said it plans to begin increasing the number of carriages on the route from three to six when its new Nova fleet is phased into service later this year.