Why was digger bucket left on busy rail line, and why did it take so long to get trains running again after one hit it?
Questions have been raised over how construction equipment came to be left on a busy railway line near Lisburn - and why it has taken more than 24 hours to restore service on a key rail route.
A Portadown to Lisburn train hit the bucket from an excavator on the line at around 6.45am.
The object smashed the two front windows of the train, but no one was injured and passengers were transferred to buses.
The incident caused widespread disruption, and traffic chaos ensued on the roads.
Translink, the Health and Safety Executive, and the UK's Rail Accident Investigations Branch are carrying out probes into what happened. A statement on the Translink website said the company hoped to restore the rail service this morning.
But speaking to the Belfast Telegraph last night, the DUP MP for Lagan Valley, Jeffrey Donaldson, said it was taking too long to get things back up and running.
"It seems to be taking an inordinate amount of time to restore the service," he said.
"This has caused major disruption not only to commuters, but also to the flagship Enterprise service."
Mr Donaldson said while he understood the need for caution, the public deserved more clarity sooner.
"With recent terrorist-related incidents on the line, the police and other emergency services had to be careful that they weren't being lured into some kind of trap," he said.
"Nevertheless, I know there were many people across Lisburn who were very worried by this incident and feared the worst.
"So while we're all relieved that in the end it didn't involve any injury or loss to human life it has caused enormous inconvenience for many people."
He added: "Questions need to be asked as to how this equipment was left on the line.
"We must understand the implications for the safety of passengers on the train and we need answers regarding the response - in terms of the amount of time it took to clear the track."
Translink chief executive Chris Conway said Translink had employed contractors to work on the track on Wednesday night.
He added that the PSNI and the Rail Accident Investigation branch would be working with Translink to investigate fully exactly what happened.
A Translink spokeswoman said safety was their "top priority".
She added: "Bus substitution services were immediately arranged for local passengers between Portadown and Lisburn and cross-border passengers were bussed between Belfast and Newry."
The line remained closed yesterday to facilitate the ongoing investigation. Translink said their employees were doing everything possible to have services resumed as soon as possible. Motorists in the area complained about heavier than normal traffic during the rush-hour commute yesterday morning.
Roads expert Dr Wesley Johnston said Northern Ireland's over-subscribed roads can't cope with even minor incidents.
"Our road system runs at or above capacity in the rush hour so even if something goes very slightly wrong it causes a delay and then as people try alternative routes there is a ripple-out effect into the surrounding areas," he added.