A train driver who sped through a 20mph (32km/h) zone at 121mph (194km/h) was distracted as he grasped for medication for an illness.
The driver missed two speed restriction warning signs, according to the Rail Accident Investigation Branch (RAIB).
An emergency limit was set at Sandy South Junction, Bedfordshire, after a crack was found on a set of points the day before the journey, on 19 October.
When he finally noticed, it was too late to reduce his speed completely.
The RAIB also said a failure by London and North Eastern Railway (LNER) to notify drivers of emergency restrictions prior to their journey was a contributing factor.
Always a risk
The driver told investigators he began to feel worse as the train got closer to Sandy and he did not notice the first warning sign.
He then misunderstood a second warning and thought it did not apply to his train.
The RAIB warned that although there was no accident on this occasion, there was always a risk when trains do not comply with speed restrictions.
No-one was injured as a result of the speeding and the LNER service from Aberdeen made it to London King's Cross safely.
The report stated it was also possible the driver missed the first warning sign because of issues with its visibility.
LNER now warns all its drivers of any speed restrictions on their routes following recommendations by the RAIB.