London Underground, Tube Lines Ltd and Schweerbau GMBH had pleaded guilty to endangering passengers and staff under Health and Safety law, after the 39-tonne maintenance wagon hurtled through seven stations before it could be stopped.
A train in front of it was told to keep going and not stop, while its passengers were told to run to the front carriages, the Old Bailey heard.
Judge Richard Hone said: "There was the potential of terrible tragedy."
He said the runaway train was out of control for four miles during 16 minutes.
"To those involved it must have seemed an extremely frightening eternity," he added.
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The 47-year-old man died shortly after 3pm on Tuesday.
Specialist explosive detection dogs searched through the town examining letter boxes and phone booths for signs of potential devices.
Temperatures reached 29.1C in St James's Park, central London.