Rail commuters suffered delays of up to an hour and a half into King's Cross today.
Packed services from Doncaster/Leeds via Peterborough to London were forced to a stop-start crawl after a train broke down and blocked the busy line near Huntingdon.
The train came to a stop after suffering problems with the hydraulic system; when that fails the brakes lock on automatically.
Engineers at the scene believe track ballast may have have been thrown up as the train sped over and which damaged the hydraulic pipes.
Attempts were made to push it out of the way using another train - but the brakes were locked on so hard that didn't work. Buses were then used to rescue several dozen passengers who by then had been stranded for two hours.
The train came to a stop at 5.30 am and was still blocking the line at 8 am.
An FCC spokesman said: "We apologise to passengers for the delays they suffered this morning. We have our engineers on site and we are working to clear the train out of the way and restore normal service as quickly as possible."
First Capital Connect (FCC) East Coast, First Hull Trains and Grand Central services each reported major delays into the peak.
Worst hit were FCC with some trains starting and terminating at Huntingdon - not running between Peterborough and Huntingdon.
It happened after the driver of the FCC 0510 Peterborough to King's Cross reported he was at a standstill with brake problems.
To make matters worse the train came to a stop where currently there are just two tracks.
Engineers were working to get the train moving or tow it out of the way.
On the Tube, Bakerloo line services were suspended between Elephant & Castle and Paddington due to a first alert at Regent's Park station.
The line reopened in time for the peak but with severe delays.
