The Hatfield train crash trial may last as much as a year, the judge has warned potential jurors.
The Hatfield crash claimed four lives
Engineering firm Balfour Beatty and five rail managers are on trial for manslaughter over the crash.
Four people died and 35 were hurt when a section of rail broke and a high speed train derailed in October 2000.
Mr Justice Mackay said the trial was expected to last between nine months and a year. Jurors are being chosen from a panel of 75.
The judge told the jurors they would be trying three employees of Railtrack and two of maintenance company Balfour Beatty.
Balfour Beatty Rail Maintenance faces a corporate manslaughter charge.
Mr Justice Mackay said: "This will be a long trial but also an important one and, I believe, an interesting one."
Balfour Beatty, Anthony Walker, its rail maintenance director and civil engineer Nicholas Jeffries, deny manslaughter.
Railtrack's Alistair Cook and Sean Fugill, both asset managers for the Londonnorth-east zone and track engineer Keith Lee, also deny manslaughter.
All five men, along with four others, are also accused of breaches of health and safety laws.
Network Rail, which took over from Railtrack, denies Health and Safetycharges.
The jury selection process in expected to last most of the week.
The trial will then transfer to another courtroom in Holborn, central London.
The accident, on 17 October 2000, happened when the London to Leeds express came off the tracks at 115 mph, when it was derailed by a cracked section of rail.
The accident on the East Coast Main Line sparked major disruption.
The overall responsibility for the line was Railtrack's - the company that has now become Network Rail.
Those who died in the accident were Steve Arthur, 46, from Pease Pottage, West Sussex; Peter Monkhouse, 50, of Headingley, Leeds; Leslie Gray, 43, of Tuxford, Nottingham; and Robert James Alcorn, 37, of Auckland, New Zealand.