Hollyoaks has been found to have breached the rules on pre-watershed violence in the episode in which undercover policeman Simon Walker died.
TV watchdog Ofcom will look at viewers' attitudes towards on-screen violence in everything from documentaries to drama, and examine whether the amount and nature of violence in soaps has changed over the last decade.
Channel 4 soap Hollyoaks breached broadcasting rules in an episode, screened at 6.30pm in March, in which Walker was killed by a speeding train following a fight with ex-drug dealer Brendan Brady.
It ended with Walker screaming as he fell backwards on to the railway track and into the path of a train before the camera cut away to show the train passing at high speed.
There were no images of Walker being hit by the train but the fight in the build-up included several blows to the face and stomach, Brendan's head being pushed through railings, Walker with bloody lips and blood running from his nose, while viewers heard the groans as they inflicted blows.
Channel 4 said that the storyline had run for around a year and that viewers were notified that a dramatic episode would be broadcast. It said that the fight scene only lasted around a minute, was at the end of the episode and carefully edited.
Ofcom said that it accepted that violence was "a part of life and integral to many dramas, including those broadcast pre-watershed."
But it said that the pre-programme information was too vague and would "not have prepared the significant number of younger viewers in the audience, or their parents, adequately for the violent, intense and shocking scenes which followed."
Channel 4 argued that Hollyoaks, which regularly deals with storylines about sexual abuse, domestic violence and drugs, was aimed at a teenage audience.
But Ofcom said figures showed that 15 per cent of the audience of the train death episode were children aged four to 15 and 10 per cent were aged four to nine.
The watchdog "concluded that cumulatively the violent content in this sequence exceeded viewers' expectations for a drama transmitted long before the watershed when young children were available to view and in this case were watching in large numbers".