Commuters travelling through London Euston faced major disruption after services were delayed or cancelled during rush hour.
All rail services from the central London station were suspended and the West Coast main line was closed while police dealt with a trespasser incident on Friday evening.
Some commuters were left in the dark after the power was shut off on trains travelling to and from the major transport hub – with staff handing out glow sticks to passengers.
Officers attended with London Fire Brigade and British Transport Police (BTP) and an air ambulance was called.
A London Ambulance Service spokesman said: “We were called at 4.29pm to reports that a person was injured near to Wembley Central railway station.
“We dispatched a number of resources to the scene including our hazardous area response team and London’s Air Ambulance.
“Crews treated a man at the scene and he was taken to hospital by road.”
Pictures taken by commuters at the transport hub show the station concourse packed with people.
Network Rail said the West Coast main line was closed for a number of hours on Friday evening while emergency services dealt with the incident.
All lines have been reopened, said Network Rail, but they warned of “serious disruption” for passengers until end of service on Friday.
James Dean, managing director for Network Rail’s West Coast main line south route, said: “Our specialist teams have been working with the emergency services to deal with a serious trespass incident this evening near Wembley Central.
“We thank people for their patience while we try to get services back up and running.”