A reward of Rs 50,000 had been declared for any information leading to his recovery, they added.
The investigation into the case was transferred to the Crime Branch by the Delhi High Court.
The child, a resident of slums near near DRP Line, Lahori Gate went missing on May 21, 2012.
Teams under the supervision of Joy Tirkey, Deputy Commissioner of Police (Crime) fanned out in all directions and checked almost every railway station on the train routes of Gorakhdham Express, Toofan Mail, Punjab Mail, Shan-a-Punjab, Howrah-Delhi Kalka Mail and Jammu Mail.
On June 20, a police personnel received information that the missing boy could be living with a family in Hussainpur, Kapurthala, Punjab.
His father accompanied the police team and the boy was found living with a family which had adopted him two years ago, said Alok Kumar, Joint Commissioner of Police (Crime).
The father recognised him because of a scar mark on the forehead and his facial features.
The three-year-old boy and his family lived in the slum cluster near Pul Mithai, Pili Kothi, near railway line where a lot of trains keep moving.
The boy had been playing with other children when he went missing. It was suspected that the boy might have climbed on a train and travelled somewhere.
About two years ago, one Sanjay, a resident of Munger, Bihar along with his wife and two children was travelling on a local train in Lakhi Sarai, Bihar.
The family offered him food and asked him about his parents. The child was unable to tell them anything about himself. The family tried to find the parents in the train but in vain. They decided to keep the child, he added.
After spending a month in Bihar, the family returned to Hussainpur in Kapurthala, Punjab where Sanjay worked in a factory. The family kept on trying to locate the boy's family but in vain. They told everyone they met to help them search for his parents.
Meanwhile, the Crime Branch had been pasting Hue and Cry Notices all over Delhi and the neighbouring states.
Sanjay's neighbour in Kapurthala, Minta Devi (55), was visiting Delhi and saw the notice with the boy's picture at New Delhi Railway Station, he said.
She took a picture of the poster on her mobile phone and showed it to Sanjay, asking him to get in touch with the Delhi Police. He called up the Crime Branch and this led to the child being traced.
The child does not remember anything for the four-year period between 2012 and 2016 before he was found by Sanjay in the local train, said the officer.
The child would be produced before the Child Welfare Committee before formally being handed over to his parents.
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