A disability care service provider says it is cooperating with a police investigation into the death of a young boy with severe autism who was hit and killed by a train in Sydney's south.

The 11-year-old boy died after he escaped from a respite care facility at Oatley just after 7:00pm yesterday.

His carers alerted authorities and a police search was set up involving Polair and the dog squad.

The boy's body was found at the Oatley train station two hours later.

Police confirmed on Monday morning that the child, who was non-verbal, was hit by a train.

Civic Disability Services Limited confirmed the child was from its short-term accommodation facility for children and young people.

It said family members had been informed of the details, but due to the sensitive nature, and out of respect for the family, it would be inappropriate to release any more details.

"This is a tragic and distressing incident and our deepest sympathies and thoughts go out to the child's family", Civic CEO Annie Doyle said.

"Counselling and assistance services have been made available to our staff and others who have been affected."

The Sydney Friendship Circle, a support group for families of children with special needs, posted tributes to the 11-year-old on social media, saying he was a treasured son, grandson, brother and friend.

"Today the world lost a beautiful angel. As we try to process this tremendous loss, your beautiful life flashes in front of us," the post read.

"We are going to miss your mischievous smile, the joy in your eyes, your curiosity about the smallest creation and your zest for life. We will miss you so much."

The boy was found dead at Oatley station two hours after he left the care facility. ( ABC News: Rachel Hayter )

The NSW Department of Family and Community Services (FACS) said it was unable to comment on the circumstances of the boy's death, as the matter was being investigated by police.

"The death of any child is a tragedy," a FACS spokesman said in a statement.

"The staff of the Department of Family and Community Services were deeply saddened to hear of the death of an 11-year-old boy yesterday and would like to extend their sympathies to his family," it said.

NSW state MP for Oatley Mark Coure offered his condolences to the boy's family, describing the incident as "just heartbreaking".

"My heart goes out to the family, friends and carer this morning," he said in a statement to the ABC.

"I want to thank the emergency services, and all the locals who last night helped to search for this little boy.

"As a father, I can only imagine the grief of the family and extend my deepest sympathies for their unimaginable loss.

"I would encourage anyone with any information regarding this ongoing investigation, to please contact the police."

Members of the public have offered prayers for the boy's family. ( ABC News: Rachel Hayter )

NSW Transport Minister Andrew Constance said his thoughts were with the family and he offered his "sincere condolences".

"This is a tragic circumstance for the local community and all those involved, including our staff at Sydney Trains," he said in a statement.

"An investigation is underway into the circumstances surrounding the child's death, and police are appealing to anyone with information to contact St George Police or Crime Stoppers."