The seven railmen wearing fluorescent uniforms were left flattened on a pavement after the taxi ploughed into them on a bridge.
The railway workers were given emergency treatment at the roadside by 999 teams.
A taxi driver was arrested at the scene on a bridge just yards from the Millennium Stadium in Cardiff. He was being questioned by police.
The railway team were struck on a pavement just 200 yards from the Welsh capital's main train station.
One witness said: "It was incredible. I saw an argument between the workers in orange and a taxi driver.
"Then that stopped but the taxi came round the corner - then the next thing I knew they were hit.
"They looked like they were just going home from work and headed to the train station."
Their injuries were described as "serious but not life threatening" after the alleged black cab attack on the bridge next to Cardiff Central Station.
The taxi had a shattered windscreen after it hit the seven men shortly after 7pm on Tuesday night.
Police were at the bedside of the seven men waiting to take witness statements about the minutes before the carnage.
Another witness said: "I turned up to see all of these people in their work clothes sprawled out on the floor I had no idea what happened.
"It looked like a couple of them might be dead. They weren't moving and it was really worrying.
"But loads of ambulances and police cars turned up really quickly and people that were passing by rushed to help."
A spokeswoman for South Wales Police said: "There was one vehicle involved and the driver was arrested.
"They had serious injuries, including spinal injuries, but none were life-threatening."
The workers were taken to University Hospital of Wales, Cardiff where they were being treated. The driver, who hasn't been named, was held for questioning.