The confusion came in west London during the evening rush hour
Two crowded London Tube trains came dangerously close to one another during the rush hour.
London Underground (LU) has begun an inquiry into the incident near Ealing Broadway in west London at about 1800 GMT on Friday.
The Piccadilly line train and a District line train came within one train length of each other after confusion at a red signal.
A signalman is believed to have been suspended following the incident.
The District train had stopped after going through a red signal but had then been given permission to move ahead.
At that moment, a moving Piccadilly line train crossed its track immediately in front.
A Transport for London spokesman said: "Safety is LU's top priority and we have an excellent safety record.
"Following an incident, involving District line and Piccadilly line trains on 27 March, an investigation has been launched by LU, and the Railway Inspectorate and the Rail Accident Investigation Branch have been notified."