12 Angry Men

In a New York City courthouse a jury commences deliberating the case of an 18-year-old boy[9] from a slum, on trial for allegedly stabbing his father to death. If there is any reasonable doubt they are to return a verdict of not guilty. If found guilty, the boy will receive a death sentence.[10]
In a preliminary vote, all jurors vote "guilty" except Juror 8 (Henry Fonda), who argues that the boy deserves some deliberation. This irritates some of the other jurors, who are impatient for a quick deliberation.
Juror 8 questions the accuracy and reliability of the only two witnesses, and the prosecution's claim that the murder weapon, a common switchblade, was "rare." Juror 8 argues that reasonable doubt exists, and that he therefore cannot vote "guilty," but concedes that he has merely hung the jury.
Juror 8 suggests a secret ballot, from which he will abstain, and agrees to change his vote if the others still agree. A new "not guilty" vote appears. An angry Juror 3 (Lee J. Cobb) accuses Juror 5 (Jack Klugman), who grew up in a slum, of personal bias, but Juror 9 (Joseph Sweeney) reveals it was he that switched, agreeing there should be some discussion.
Juror 8 argues that the noise of a passing train would have obscured the verbal threat that one witness claimed to have heard the boy make to his father, and notes that such threats are also rarely sincere. Juror 5 then changes his vote. Juror 11 (George Voskovec) also changes his vote, believing the boy would not likely have tried to retrieve the murder weapon from the scene if it had been cleaned of fingerprints.
Jurors 5, 6 and 8 question the witness' claim to have seen the defendant fleeing 15 seconds after hearing the father's body hit the floor, since he was physically incapable of reaching an appropriate vantage point in time. Juror 8 states that he merely assumed it was the defendant running. An angry Juror 3 shouts that they are losing their chance to "burn" the boy. Juror 8 accuses him of being a sadist, not interested in the facts of the case. Juror 3 shouts "I'll kill him!" and lunges at Juror 8, but is held back. Juror 8 says, "You don't really mean you'll kill me, do you?" proving his earlier point.[6] Jurors 2 (John Fiedler) and 6 (Edward Binns) then change their votes, tying the vote at 6â6.
Juror 4 (E. G. Marshall) doubts the boy's alibi, because he could not recall it in much detail. Juror 8 tests how well Juror 4 remembers previous days, which he does, with difficulty. Juror 8 asserts that the boy would have been under significant emotional stress when questioned, and his recall would have been significantly worse.[11]
Several jurors then change their votes when Juror 5 relates growing up amidst knife fights in his neighborhood, and states that the boy could not have stabbed his father in a downwards motion, as the prosecution asserts, since his father was significantly taller.
Increasingly impatient, Juror 7 (Jack Warden) changes his vote to hasten the deliberation, which earns him the ire of other jurors for voting frivolously. Juror 7 eventually admits that he now truly believes the defendant is innocent.[12]
Jurors 12 (Robert Webber) and 1 (Martin Balsam) then change their votes, leaving only three dissenters: Jurors 4, 10 (Ed Begley) and 3. Juror 10 then vents a torrent of condemnation of slum-born people. All but one of the jurors successively turn their backs on him as his diatribe wanes, leaving only Juror 4, who tells him not to speak again.
When the remaining "guilty" voters are pressed to explain themselves, Juror 4 notes the woman who saw the murder from her bedroom window across the street. Juror 12 then reverts his vote, making the vote 8â4.
Juror 9, seeing Juror 4 rub his nose (which is being irritated by his glasses), realizes that the woman who allegedly saw the murder had impressions in the sides of her nose, indicating that she wore glasses, but did not wear them in court. Juror 9 says that she would not have been wearing them while trying to sleep, and points out that on her own evidence the attack happened so swiftly that she wouldn't have had time to put them on. Jurors 12, 10 and 4 then change their vote to "not guilty", leaving only Juror 3.
Juror 3 gives a long and increasingly tortured string of arguments, building on earlier remarks that his relationship with his own son is deeply strained, and a resulting wish to find the defendant guilty. He finally breaks down and tears up a photo of him and his son, and changes his vote to "not guilty", making the vote unanimous.
As the jurors leave the room, Juror 8 helps the distraught Juror 3 with his coat in a show of compassion. The film ends when the friendly Jurors 8 (Davis) and 9 (McCardle) exchange names, and all of the jurors descend the courthouse steps to return to their individual lives.

Question: What creates the noise that Juror 8 argues would obscure the verbal threat made from the boy to his father?
Answer: A train