In contrast to vertical slats and bars that signify guilt, round signifies innocence in this film (as in the double, round collars that Babs wears), plus, Hitchcock uses light to make Guy's wrist buttons shine brightly. We know by this that Guy's hands are good. They are not the hands of a murderer. He is the innocent man, wrongfully accused and working to clear himself.
At a party at Senator Morton's house, during a discussion about murder, Bruno coaxes Mrs. Cunningham, an older woman, to allow him to put his hands around her throat. She is foolishly flattered by his attention and actually lets him. Ann's younger sister Babs happens to come near and when Bruno sees her, we see Babs through Bruno's eyes. She wears glasses like Miriam did (double lenses) and in the lenses of her glasses two flames appear -- the flame of the cigarette lighter, doubled. It is the second most powerful of all the doubles in the film. Bruno nearly kills Mrs. Cunningham; she is frightened out of her wits. We know that Bruno is truly psychopathic and that he is re-experiencing strangling Miriam (having a flash-back) as he looks at Babs and the double flames in her glasses. The scene also suggests he enjoyed killing Miriam. Babs looks something like Miriam did and represents Miriam's double. But Babs is only about 16 years old and innocent. This double image also makes the viewer afraid for Babs' safety! We see that Bruno is very capable of killing again, and something violent is definitely going to happen soon.
The doubles often come in the form of stripes and bars. There are bars, for example, on the billboard entering the amusement park, dark pillars at the tennis courts, stripes on the awning behind the game, woodwork in Bruno's house and in the senator's, spokes in the banister on the stairway at Bruno's, the stairs on the map Bruno drew, and double banks of narrow window panes in silhouette. During the night scene at Bruno's house, when Guy goes to talk to Bruno's father, viewers are bombarded with images of stripes and bars. This implies there is something prison-like about Bruno's home -- his father concerned but his mother in denial -- that the whole atmosphere of the home has produced a criminal. Guy and Ann wear stripes, too, but theirs are cheerful -- Ann's tucked dress, for example, Guy's striped tie, striped pajamas, and striped tennis sweaters. Cheerful stripes signify their law-abiding natures. Bruno's stripes are always dark and shadowy signifying his criminal character.
Once he finally realizes that Guy is not going to keep his end of the bargain (and kill Bruno's father for him), Bruno goes back to the amusement park. He intends to leave Guy's cigarette lighter there and "frame" him for the murder of Miriam. When Guy enters the amusement park to interrupt Bruno's plan to plant evidence at the crime scene, we know there will be a confrontation between the two men because we see two young children with their mother in the middle, each child carrying a helium balloon. Because balloons are the playthings of children, and in the image they are flying high (like hope), we can interpret the message that good will triumph over evil in the final confrontation, and Guy's innocence will soon be revealed. The balloons also represent Guy's hope that he will prevail.
Earlier, Bruno accidentally dropped Guy's lighter down a sewer grating at the park. Three men come and cheer him on as he makes efforts to get it out. As Bruno reaches down the drain to retrieve it, we see in several suspenseful shots the lighter lying there with Bruno's hand trying to reach it -- and also a rectangle, similar in size and shape to the lighter itself, in the left foreground at an angle. Unlike the shining double-image balloons, however, this double (dwelling in the storm drain), is rather murky and weak in color -- perhaps signifying that evil is really weak and cannot ultimately prevail.
Another disturbing double image is the horses' feet on the merry-go-round coming down over and over again as Bruno and Guy struggle with each other in the wild out-of-control merry-go-round scene. In the struggle between good and evil, the horses' feet seem more evil than good as Bruno tries to kill Guy. That Guy is good and that this is a struggle between good and evil is also strengthened when Guy stops defending himself against Bruno and saves a child whom Bruno knocked off the horse and placed in danger of falling into the machinery of the merry-go-round. The chaos of the whirling merry-go-round and the people screaming in terror and pain echoes the depth of the life-and-death struggle between Guy and Bruno. When the merry-go-round finally stops, Bruno is crushed under the machinery. As he lies dying, unrepentant and still a liar, his head is momentarily shown between double vertical slats or bars, (boards on the floor of the merry-go-round?) one on each side very close like prison bars, signifying that justice has come and crime has been brought under control.
It is important to note that although Hitchcock portrays Guy as basically a good and honest person, Guy is not entirely "perfect" in character. He did get very angry when Miriam double-crossed him, and he said forcefully that he wished he could kill her. "I could strangle her!" he said. Although he is innocent physically of doing anything to cause Miriam's death, mentally he wished her dead. So, he is not entirely without "sin." He benefits from Miriam's death, too, because it leaves him free to marry Ann Morton, the woman he loves. Perhaps the double image of his tennis rackets, crossed on the silver lighter, imply this also -- innocence and guilt, both in the same person.
It has been said that films are the collective dreams of society and thus portray society's repressed desires, fears, and unacceptable impulses. Strangers on a Train could be interpreted as a dream in which Bruno represents the unacceptable part of Guy's psyche, the part that gets enraged and wants to kill another human being. Bruno's act -- murdering Miriam -- externalizes Guy's wish to kill her and be rid of her. After seeing Miriam, Guy tells Ann over the phone, "I could strangle her!" The next moment we see the double-image of Bruno's hands close-up as he flexes them threateningly. The close juxtaposition implies a connection between Guy's desire to kill Miriam and Bruno's ability to fulfill that desire. When Guy first learns about the murder and is horrified, Bruno says, "But Guy, you wanted it!" Psychologically, one could see the basic conflict as Guy's struggle to preserve his identity -- not to lose it to Bruno like the good Dr. Jekyll lost his identity to the evil Mr. Hyde. In any event, Hitchcock often uses the double images to express Guy's thoughts, feelings, and perceptions. For example, bars around Bruno represent Guy's need to control his unacceptable impulses. Psychologically, Bruno represents a Hitchcock commentary on primitive subconscious impulses and destructive tendencies present in human nature.
The double images in Strangers on a Train function, then, in three ways. First, they signal conflict about to occur in the story and show us things about the characters; second, they represent the protagonist's inner thoughts, fears, and desires; and third, they represent Bruno as part of the dark side of Guy's psyche. Undeniably, the story is more like a dream than real life, and yet viewers believe from the start. The film's near-obsessive focus on 2s and doubles helps to create a sense of psychological reality that springs from unreality. Only Hitchcock could have made this film. It expresses his vision of the world and his view of human nature in a unique and memorable…