The Woman in Black

In an English village, Crythin Gifford, in 1889, three young girls are having a tea party with their dolls. They suddenly look up at something off-screen and, as though hypnotized, jump to their deaths from the bedroom window.
Some years later, in Edwardian era London, widowed lawyer and father Arthur Kipps (Daniel Radcliffe) is instructed to visit Crythin Gifford to orchestrate the sale of Eel Marsh House, an estate on the marshland, and retrieve any relevant documents left by the deceased owner Alice Drablow. Upon arrival, Arthur finds many of the villagers rather unwelcoming, though he finds sympathy in a wealthy local landowner Samuel Daily (CiarÃ¡n Hinds).
The next morning, Arthur goes to meet his legal contact, Mr. Jerome, who tries to hurry him away from the village. Arthur, undeterred, travels to Eel Marsh. During his initial visit to the house, Arthur is distracted by odd noises, a bolted nursery, and the appearance of a spectral entity in funerary garb. He hears sounds on the marshes of a carriage in distress and a screaming child, but sees nobody on the causeway. He later attempts to alert the village constable, who dismisses him. Two children enter the station with their sister Victoria, who has ingested lye, and subsequently vomits blood, dying.
That night, Sam reveals that he and his wife Elisabeth (Janet McTeer) lost their young son to drowning. Elisabeth suffers from fits of hysteria, which she attributes to her boy speaking through her. When Sam attempts to drive Arthur to Eel Marsh the next day, a fleet of local men attempt to drive him off. Victoria's father blames Arthur for his daughter's death, as Arthur saw "that woman" at Eel Marsh.
At the house, Arthur uncovers correspondence between Alice and her sister Jennet Humfrye (Liz White). In her letters, Jennet denies Alice's verdict that she is "mentally unfit" and demands to see her son Nathaniel, whom the Drablows formally adopted and barred her from contacting. A death certificate reveals that Nathaniel drowned in a carriage accident on the marsh. Jennet blames Alice for saving only herself and leaving Nathaniel's body in the marsh. Jennet hangs herself in the nursery, vowing never to forgive Alice. Arthur also sees visions of dead children in the marshes, Victoria among them.
Arthur finds the nursery no longer locked. Inside, he has a vision of the Woman in Black hanging herself, alarming him. In town, Jerome's house catches fire with his daughter still inside. When Arthur attempts to save her, he sees the Woman in Black goading the girl into immolating herself. The townspeople blame Arthur for this death as well.
At her son's grave, Elisabeth tells Arthur that the Woman in Black is Jennet, who claims the souls of the village children by having them take their lives in penance for her own son being taken. Through another episode of delirium, she informs him that his son Joseph, who is coming to Crythin Gifford that night, is Jennet's next victim. In an effort to lift the curse, Arthur and Sam find Nathaniel's body in the carriage on the marsh, and place it in his nursery, where Arthur lures Jennet to him. Arthur and Sam bury Nathaniel with Jennet, though her voice echoes through the house that she will never forgive the wrongs she suffered.
Assuming Jennet pacified, Arthur and his son Joseph meet at the railway station. While bidding farewell to Sam, Arthur sees the Woman in Black lure Joseph onto the tracks towards an oncoming train. Though he attempts to save him, both Arthur and Joseph are killed by the oncoming train while a horrified Sam sees the spirits of the village children, and the Woman in Black.
After the train passes, Joseph spots a woman on the tracks, and Arthur identifies her as his late wife, the family now happily reunited. The Woman in Black watches them walk away, before sharply turning to face the audience, at which point the screen turns black.

Question: What do Arthur and Sam find in the carriage?
Answer: Nathaniel's body