Balfour Beatty went to the Court of Appeal this week to challenge a £10m fine that it received over the Hatfield rail disaster in 2000.
In the initial judgment last October, the engineering giant was fined £10m and Network Rail £3.5m for industrial negligence.
Balfour's lawyers are now arguing the fine should be reduced because it had pleaded guilty to breaching the Health and Safety at Work Act.
Balfour was responsible for track maintenance on a section of damaged track that had been identified 21 months before the tragedy, but not replaced.
Balfour's fine was about five times greater than the previous highest fine for health and safety breaches handed out in an English court.
At the time of the judgment, Mr Justice Mackay described the case as "one of the worst examples of sustained industrial negligence".
Four people died and 102 were injured in the Hatfield crash when a train derailed at 117 mph.