Armed police were sent to Hackney Central station on Wednesday (March 1) following a report of a “firearms incident”.
Police were called at 1.29pm and a man was found with “laceration wounds” and was taken to hospital where he remains in a critical condition.
The station remains closed while investigators make enquiries.
British Transport Police (BTP) said detectives are keen to speak to anyone who was in the area at the time of the incident.
Witnesses, or anyone with information, can contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 334 of March 1. Alternatively, call Crimestoppers anonymously on 0800 555 111.
A London Ambulance Service spokesperson said: “We were called today at 1.24pm to reports of an incident at Hackney Central station, Hackney.
“We sent a number of resources to the scene, including an incident response officer, two ambulance crews and a medic in a fast response car.
“We also dispatched London’s Air Ambulance. Our first medic arrived within three minutes.
They added: “We treated a man at the scene and took him to a major trauma centre as a priority.”
There remains a significant police presence at the station, with officers combing the grassy banks for evidence.
BTP said in a statement: “Armed officers were called to Hackney Central Overground station following reports of a firearms incident at 1.29pm on March 1.
“A man was located at the station with laceration wounds and was taken to a local hospital where he remains in a critical condition.
“The station remains closed while officers make enquiries into the incident.
“Detectives are keen to speak to anyone who was in the area at the time of the incident, as they may have information which could help their investigation.
“Witnesses, or anyone with information, can contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 334 of 1 March.
“Alternatively, call Crimestoppers anonymously on 0800 555 111.”
Around 10-20 police officers are on the scene.
Hackney Council called for people to avoid the station.
TfL said on Twitter that Hackney Central station will remain closed while officers investigate.
Overground services are still stopping at Hackney Downs.