The firm apologised to East Coast customers after publishing punctuality figures that showed it was to blame for 70% of delays on the line, which runs between London and Edinburgh.
Around one in six East Coast trains were late between August 18 and September 14, making the nationalised network the least punctual in the country during that period.
Nationally, Network Rail (NR) was to blame for six out of every 10 (60%) delayed services as punctuality on the railways tumbled to 92.3% from 94% in the same period last year.
The company said: "Network Rail would like to apologise to East Coast's customers for the disruption caused by these infrastructure failures and the company is redoubling its efforts to improve the reliability of this critical piece of railway infrastructure."
The firm said deaths on the line and other external factors made up a fifth of the delays it was to blame for on East Coast services.
The biggest failures related to overhead wire failures in three separate incidents, it said.
Network Rail is responsible for maintaining Britain's rail tracks, signalling, bridges, tunnels, level crossings, viaducts and 17 stations.
Of the 60% of train delays Network Rail was responsible for nationwide, external factors such as weather, trespass and vandalism, and cable theft were to blame.
The most punctual train operator nationwide was c2c, which runs trains through Essex into the City of London. Nearly all (98%) of its trains were on time.
A train is deemed to have arrived on time if it arrives within five minutes of its expected time for commuter services and within 10 minutes for long-distance services.
The annual average of trains arriving on time now stands at 90.7%.
These are the punctuality figures for each train operator between August 18 and September 14:
:: Arriva Trains Wales - 93.2%:: c2c Rail - 98.1%:: Chiltern - 96.6%:: Crosscountry - 90.3%:: East Coast - 83.4%:: East Midlands Trains - 91.3%:: First Capital Connect - 86.7%:: First Great Western - 90.4%:: First Scotrail - 93.2%:: First Transpennine Express - 88.7%:: Greater Anglia - 93.0%:: London Midland - 88.4%:: London Overground - 96.1%:: Merseyrail - 97.2%:: Northern Rail - 93.1%:: Southeastern - 94.5%:: Southern - 89.0%:: South West Trains - 94.5%:: Virgin Trains - 88.4%
A spokesman for the Association of Train Operating Companies (ATOC) said: "More than nine out of ten trains arrived as planned according to the Government set measure for punctuality. Train companies will continue to work with Network Rail and focus efforts on providing passengers with an even better service."