An octogenarian suffering from Alzheimer’s miraculously survived a close encounter with a speeding No. 4 train after he wandered onto the tracks – walker and all – this afternoon.

Anatoliy Zilberman, 84, lost several toes off his left foot and suffered severe bruising, when the train nearly collided with him on the express tracks just north of Union Square, police said.

The incident snarled subway service for several hours.

Police are unsure how Zilberman, who lives in an assisted living facility on East 23rd Street, ended up in the tracks.

Zilberman was taken to Bellevue Hospital and is currently in the operating room, hospital staff said.