Trains to London St Pancras from Chesterfield are experiencing long delays.
East Midlands Trains said that there are 30 to 40 minutes of delays expected to its Sheffield to London St Pancras train.
The line passes through Chesterfield, Belper, Derby and Long Eaton.
There is currently a diversion in place at Leicester because of an "safety inspection".
A East Midlands Trains spokesman said: "A broken rail is affecting our southbound trains from Leicester, trains will be diverted where possible between Leicester and Kettering, this will add up to 30-40 minutes delay and our trains will not stop at Market Harborough.
"We have ticket acceptance on alternative routes, if you are travelling into London, from Sheffield / Derby / Nottingham and Leicester, tickets will be accepted via LNER. Virgin Trains and Cross Country UK."
UPDATE Friday, March 29: "Network Rail have completed repairs at Kilby Bridge and services have started moving again between Leicester and Kettering."