Factors causing the landslips included "absent or ineffective" drainage, said the report from the Rail Accident Investigation Branch (RAIB).
Many Network Rail (NR) earthworks were constructed with steeper slopes - leading to a greater likelihood of landslips - than would be achieved with modern design procedures, the RAIB added.
The report went on: "NR's on-going earthwork improvement programme is unlikely to achieve modern criteria in the foreseeable future."
The RAIB said: "The landslips were caused by factors including heavy rain, absent or ineffective drainage and activities undertaken, or not undertaken, on neighbouring land.
"In several instances trains were being operated without special precautions when there was a significant risk of encountering a landslip."
The report continued: "RAIB has found that, in some circumstances, key information provided by specialist staff examining earthworks is not considered when the slope management strategy is determined during evaluation.
"There is a lack of clarity about who should be carrying out visual checks for risks which can develop on neighbouring land between examinations which take place at intervals of up to 10 years. The mandated process for collecting information about neighbouring land is, in parts, difficult to implement and not usually followed."
The RAIB made five safety recommendations.
These were the six incidents looked at in the report:
June 28 2012 - Landslip at Loch Treig, near Tulloch in Scotland. Freight train derailed. No injuries;
July 18 2012 - A First ScotRail passenger train going from Oban to Glasgow struck landslip debris near Falls of Cruachan but did not derail. There were no reported injuries but the train suffered minor damage. The report said: "If the train had been derailed by the landslip debris, it is possible that it would have fallen down an adjacent slope."
July 18 2012 - A First ScotRail service from Newcraighall to Edinburgh was derailed when it struck landslip debris between Rosyth and Dumfermline. There were no injuries. The track and train suffered minor damage.
August 30 2012 - A Northern Rail service from Maryport to Lancaster derailed when it hit a landslip between St Bees and Nethertown on the Cumbrian coast. There were no reported injuries and only relatively minor damage to the train. The report said: "If the train had derailed to a greater extent, it is possible that it would have fallen down an adjacent slope."
January 30 2013 - An Arriva Trains Wales service from Rhymney to Barry Island struck a tree which had fallen on to the track after a landslip on the approach to Bargoed station. The leading bogie derailed, the train suffered minor damage and a passenger was taken to hospital suffering from shock;
February 11 2013 - A train driver noticed a track defect when passing Hatfield Colliery, near Stainforth, South Yorkshire. The line was not fully opened again until July 8 2013.