Law & Order: UK (Series 7) (2013)

A car left on a level crossing causes a crash which kills fifteen people though the driver cannot be found. Initially the police, led by Natalie Chandler's replacement, the recently promoted Wes Leyton, suspect the car owner, the unpleasant Michael Gennis, but learn that the car was stolen and the actual driver, Finn Tyler, intended to kill himself by driving onto the track but survived. Jake Thorne is anxious to prosecute for murder but Tyler's lawyer, Kate Barker, puts up a case of diminished responsibility by reason of insanity though the case is found. Shortly afterwards Tyler is found dead in his cell and suspicion falls on D.S. Sam Casey, being his last known visitor. Jake also learns that Kate is to join the prosecution team.