A man who fell from a bridge near London Euston – bringing the station to a standstill – had fled from police minutes earlier.
Officers were called to the railway terminal to reports of a passenger behaving aggressively on board a train.
When it arrived, the man “made off”, triggering a search, British Transport Police (BTP) said.
He fell from the bridge about 50 minutes later and sustained “serious injuries”, the force added.
There were reports of a trespasser on the tracks after police were called at about 17:00 GMT on Thursday.
Large crowds of commuters gathered at the station while services were halted.
A BBC journalist travelling from Euston said their train manager announced the delays were “caused by a man who ran on to the track at Euston, climbed up a bridge and then jumped on to the top of a train”.
The train manager said the man had suffered an electric shock and was in hospital, they added.
National Rail said power to overhead lines was switched off while emergency services attended the scene.
All lines have since reopened, it added, but warned of possible delays and cancellations.
Virgin Trains, West Midlands Trains and London Overground services operate at Euston.
Virgin said it expected to operate a full service on Friday.
BTP said the incident had been referred to the Independent Office for Police Conduct.
Commuters caught up in Thursday’s delays complained of long waits and crowds.