A train driver passing the scene of one of the worst British motorway crashes in living memory described the fog on the night as "weird".
Andrew Gregg was driving a Cross Country service from Exeter to Bristol when he passed through the Taunton area about 20 minutes before the crash.
The rail line runs close to the M5 motorway where seven people died and 51 were injured during a series of crashes between 34 vehicles on the northbound carriageway near Junction 25 on the evening of November 4 2011.
Motorists have told of entering a wall of thick fog - described by some as a "white curtain" or "emulsion" - and were unable to prevent multiple collisions. Other drivers have described smelling smoke or gunpowder on the motorway.
The collision happened at 8.20pm just five minutes after a £3,000 fireworks display concluded just 200ft away at Taunton rugby club.
It claimed the lives of seven people, including grandparents Anthony Adams, 73, and Pamela Adams, 70, from Newport in South Wales.
West Somerset coroner Michael Rose resumed the hearing at Shire Hall, Taunton following the trial last year of firework contractor Geoffrey Counsell.
Mr Counsell, 51, who had been operating the display at the rugby club, was cleared at Bristol Crown Court of breaching health and safety laws on the night of the accident.
Giving evidence on the fifth day of the inquest, Mr Gregg told the court that during the journey there was fog from Exeter until it cleared at Bridgwater.
"The fog was so thick, you really couldn't see anything. I have not seen a fog like that in the job I am doing for a long time," he said.
"It's weird. Its foggy and you run into stations and it starts to clear but when we got to Tiverton it was still really foggy."
Mr Gregg, from Gloucestershire, who has been a train driver for nearly 20 years, said it was the sort of fog you only saw once in every five or 10 years.
He added: "You do get fog down there but not as bad as that night in question."
The inquest also heard from spectators watching the display at the rugby club.
Among them was Sarah Silverwood who was stood in the club's main stand with her husband and two children. She described seeing a "wall" of smoke drifting across the rugby pitch.
Mrs Silverwood said that when they arrived the visibility was good, apart from a low mist on the ground, but were still able to see the M5 and the railway line.
She described seeing what she thought was smoke drifting across the rugby pitch from the Les Phippen Memorial Stand, from where Mr Counsell was operating the display.
"It was a beautiful display - it was what you would want a firework display to be," she said.
"During the display what we assumed to be smoke because it was coming from the area where they were being set off from, started to build up into a wall.
"This wall started to move across the pitch. It was almost as if the pitch immediately disappeared and by the end of the display it reached the crowd and you were sort of in it."
Mrs Silverwood said the smoke was at the same height of as the 30ft high main club stand.
"Once you were in it, you couldn't see it but obviously when it was coming towards you the height was the same as the stand we were in," she said.
Mrs Silverwood said that because of the density of it she and her husband each grabbed the hands of their children so that they would not lose them as they walked back to their car.
Another spectator was Spencer Smith, who was also at the display with his family.
He said: "At the start you could see the fireworks going up from where they were set off. About five or 10 minutes in I turned to my brother and I said 'You can't see them going up'.
"It was such a good display with so many going off that I thought it was the smoke from the fireworks that was obstructing the view. You could see them higher up, just not from where they were launching."
Describing the fog, Mr Smith said: "I just took it that there was a general build-up of smoke in that area obscuring your view.
"It was just a 'whiteness' on the pitch. I decided it was just the general smoke from the fireworks from the position from where they were being set off from."
Mr Smith said that although there was fog, he could still see the club's main stand and did not believe the fog was drifting.
Pc Richard Sims, who was an observer on a police helicopter, told the inquest that the crash scene was covered in a thick layer of fog when he and two colleagues flew over the site 45 minutes after the incident.
"I could make out the lights from the scene," said Pc Sims. "However, the scene itself was totally obscured by a thick layer of fog."
He said the fog was up to 200ft (61m) high and stretched to almost a mile (1.6km) north of the accident site and was on both sides of the motorway.
The helicopter was unable to land on the M5 so landed instead near Junction 24 at North Petherton.
A helicopter from RAF Chivenor was also in the area and came to the same conclusion that it was impossible to land on the M5 near Taunton.
Quizzed on what caused the visibility to be so poor, Pc Sims said: "The bank I saw was fog. It was rolling forward, extending its coverage.
"It was pretty bad that night."
Julian Graham, a rugby coach at the club, said he felt the display was too big for the location as it was, in his view, producing too much smoke.
"The fan part of the display started making a large amount of smoke. They were being fired at three levels - at a low level, medium level and a high level," he said.
"I remember saying to my wife that I couldn't see the lower level and could barely see the medium level due to the amount of smoke being produced."
Mr Graham said he could smell the "distinctive smell of fireworks" and added: "My opinion was that the smoke was moving more to my left - towards the motorway.
"In my opinion I felt that the fireworks display was too large and in the wrong place because of the amount of smoke it was generating.
"You have a railway track behind the club and a motorway to the left of it and it seemed to me not a good place to have it."
The inquest was adjourned until Monday.