How do trams get derailed and why does it take so long to fix?
It was a commuter's nightmare. Today the Metrolink network was thrown into chaos when a tram came off the tracks.
Passengers were forced to walk down the line following the derailment at Cornbrook on the outskirts of Manchester city centre at around 8am. Services were cancelled and disruption lasted for hours as engineers worked to repair the damaged tram.
But how do trams come off the tracks? And why does it take so long to get them up and running again?
READ MORE: READ MORE: LIVE: Tram derails in Manchester city centre with Metrolink chaos to last HOURS - updates
Firstly it's important to say derailments on Greater Manchester's Metrolink are rare. The Rail Accident Investigation Branch lists only three in its investigation archive.
And the reasons vary. In June 2008 several passengers suffered minor injuries when a tram came off the track in St Peter's Square.
An investigation found that was largely caused by the poor condition of the track, with Metrolink rebuked for 'allowing tram traffic to continue in the city centre when the track was beyond acceptable maintenance limits'.
Around 18 months earlier a tram came off the tracks at Pomona. This time there were no injuries, but like today passengers were forced to walk along the track to safety.
Services were suspended for two days while repairs were carried out. Again investigators found the derailment was caused by the 'poor condition' of the track.
And 2015 a tram came off the track in Chorlton shortly after vandals pelted it with stones, causing passengers to dive for safety.
Other reasons for trams coming off the track include driver error, debris on the tracks and silt in the tracks which can affect points and how they operate.
In the UK's worst tram accident in recent memory, seven people were killed and 61 injured when a tram overturned in Croydon, South London in November 2016. Operators Transport for London and Tram Operations Limited were later fined a total of £14m after serious health and safety failings were uncovered.
An investigation found the tram was being driven at three times the speed limit when it entered a sharp turn and toppled over.
But why does it take so long to get things up and running again? Firstly you have to ensure everyone's safety, from passengers to staff and anyone else in the immediate vicinity.
Then there's the matter of the power supply, with the Metrolink's overhead lines fed by 750 volts of electricity, more than three times the mains voltage in a home. And the complexity of getting the tram back on track also depends on the extent to which it derailed.
If it's only a few centimetres off the track, that's a lot easier to deal with than if it's a few metres off.
Transport for Greater Manchester said it would be a number of days before the reasons for today's derailment were understood.