The Rail Accident Investigation Branch (RAIB) said the incident earlier this month “could potentially have been much worse” had it led to a collision with another train.
A preliminary rail accident investigation has established that the train's braking performance was “well below normal” as it approached the capital with the driver having no control of the brakes on the coaches.
The report found that a brake pipe isolating valve was in the closed position when the train left Carstairs station.
It meant the only working brakes were on the engine "which were insufficient to maintain control of the train,” the RAIB said.
The train was eventually brought to a standstill by an emergency device being used in one of the coaches by the train manager, which caused the train brakes to apply.
The Northbound Lowlander service from London Euston terminated 650 metres east of the station following the incident at around 7.26am on 1 August. There was no damage or any injuries.
The RAIB said a full investigation into what happened "will identify the sequence of events that led to the incident.”
It will consider various factors, including how the isolating valve was closed at Carstairs, and the preparation of the train at Carstairs, including the attachment of the locomotive and testing of the train's brakes.
It will also examine how the train was driven, the suitability of relevant procedures and working practices, and the design and approval of the brake system fitted to the Mark 5 coaches