Govia Thameslink Railway (GTR) has been fined £5 million by the rail regulator over its poor communication with passengers during the introduction of a new timetable in May last year.
The Office of Rail and Road (ORR) said GTR "failed to provide appropriate, accurate and timely information" amid severe disruption on its Thameslink and Great Northern routes for eight weeks following the launch of the May 2018 timetable.
Some trains were permanently removed from the timetable but this was not made clear to passengers for several weeks, according to the ORR.
Other trains were removed or cancelled on a daily basis, leading to a "severe lack of certainty for passengers up until the point of travel", the ORR said.
The investigation also found that inadequate internal communication within GTR often left station staff with "little or no information" to help passengers.
These failures left passengers with "very little notice or certainty" about what trains would run.
A separate ORR investigation into Northern, which operates services across northern England, found in many cases passengers were given "inadequate information" in the two weeks that immediately followed the timetable introduction.
But the regulator concluded that the firm "subsequently took reasonable steps" to rectify this, meaning that no further action will be taken in relation to the issue.
The ORR has written to all train companies and Network Rail to require them to review their crisis management plans and ensure appropriate arrangements exist for assisting passengers with disabilities in times of disruption.