Different person: Tim Coulson still carries the mental scars from the events of July 7, 2005, left, and has been unable to work again  In the reflection of the television screen, I can see that tears are rolling down Tim Coulson's cheeks. We are sitting side-by-side in a sunny conservatory at Tim's home, near Henley-on-Thames, watching a documentary about the bomb that killed seven people at Edgware Road Tube station on July 7, 2005.  In this Channel 4 film, due to be broadcast for the third anniversary of the London bombings, Tim gives his first detailed account of the day when middle-aged businessman Stan Brewster died in his arms and he saved the life of a young Australian office worker, Alison Sayer.  "I went back along the same Tube route three months after it happened," Tim says calmly. "It was very difficult, but it wouldn't have been right to stay away. I couldn't bear to let the bombers win."  He is a kindly-looking, bespectacled man dressed in loose summer clothing, who is able to draw the occasional smile as he talks about helping wounded passengers, several of whom are now his close friends.  Before the radical Islamist Mohammed Siddique Khan boarded a train with a rucksack full of explosives and blew up the carriage, 53-year-old Tim loved his job as an art and design teacher at a local school.  He did not have to come to the rescue of people in the bombed train that day. He could have run back to his family and home like hundreds of others, but Tim made a split-second decision to respond to the agonised screams of injured passengers. The scenes of carnage he saw in that dark tunnel have left him severely traumatised – and unable to work as a teacher again.  The early-morning trip into London on July 7 was an unusual journey for him. He was going to the launch of a teacher training course near Farringdon, when he boarded an eastbound Circle Line train at Paddington that would soon pass through Edgware Road.  The carriage was packed to bursting point. "I can remember there was a spirit of happiness that morning. We had just found out that London had won the Olympic bid and everyone was smiling as they read the papers over each other's shoulders."  As the train passed alongside another train, there was a deafening blast. Choking, black smoke filled the carriages, the lights went out and a sickening smell wafted through the claustrophobic air. "It was quite an enormous bang, followed by showering glass hitting our carriage. I can still taste that acrid smell," he recalls. "In that moment, you subconsciously check whether you still have legs and arms. We just didn't know what was happening."  As people on the train began to panic, a Tannoy message from the driver asked those with first-aid knowledge to head to the back of the train.  Out of hundreds of commuters, Tim was one of three people who went to find out if anyone had survived the explosion. "People say, 'I can't believe you made that choice', but it wasn't a choice for me. It must be something to do with our human make-up. In no way have I ever felt any blame towards people whose instinct was to get away."  Tim realised the screams were coming from another Tube train just metres away. Along with two other passengers – later known to him as Canadian writer Peter Zimonjic and arts graduate Susanna Pell – he used an emergency ice pick to shatter the glass window of the carriage, unprepared for the bloodbath they would find on the other side.  The rescuers called out to people, but no one could hear because their ear drums had been perforated by the blast. "It was very difficult in the darkness," he remembers. "We didn't know where parts of people's bodies were." Although partially sighted, having lost an eye to cancer, Tim made his way in the pitch-black, past the bodies, to a man whose upper body was protruding from a crater in the floor of the train.  His eyes were open, Tim says. He was definitely alive. Unable to free him from the hole, Tim went underneath the train to free his legs. But the man's legs were not there – they were lying on the ground.  "The bottom of his body had been severed from the top half. He died in my arms. I lowered his body to the tracks, closed his eyes out of respect and said a short prayer."  Tim later discovered the man's name was Michael "Stan" Brewster, a businessman of the same age, who was also travelling into London for a conference. "The parallels between us were very sad," he says, shaking his head.  At this point, Tim heard the screams of a young woman lying on the tracks. Alison Sayer had been thrown from the carriage and knocked unconscious. As she begun to come round, he came to her rescue.  "I told her my name and I did my best to keep her talking. We swapped stories about holidays – anything to distract her from the reality. I just wanted to make sure this one person didn't die if I could help it.  "Her leg was seriously injured, she was losing a lot of blood and her eye was swelling up," he continues. "I was aware that her life was in danger, but I didn't want to tell her the truth."  Your mind races through every possible scenario in such a situation, he says. Was it a train crash? Had the tunnel collapsed? Could it have been a bomb? Would another blast be activated when the rescuers arrived, like the IRA bombs of the 1970s?  He thought, too, of his wife, Judy, and their three grown-up children, but forced himself to put all his energy into saving the life of the young woman in his arms. It took an hour from the time of the blast for the emergency services to arrive, but finally Tim saw the torchlight of a paramedic.  "I remember coming up the stairs at Edgware Road into the sunlight with Alison's oxygen cylinder over my shoulder, carrying one end of her stretcher. I don't remember feeling relief. I don't remember feeling anything."  When he reached the hospital, a doctor helped Tim phone his school, who immediately got in touch with his wife. Beyond that, most things are a blur. "People ask how I felt after it happened. I honestly can't tell you because I hardly remember what happened for months."  Alison's sight and leg were saved by doctors, but she decided to head back to Australia to get away from the memories of London. Tim and Alison are now very close friends, keeping in touch on the phone and by email. When he was given a bravery award from the Royal Humane Society in May, Alison sent an open letter that makes Tim cry every time he reads it.  "He didn't have to venture into our Tube," Alison writes. "He didn't have to continue to help after seeing the scene in our carriage and after trying to help another person who died in his arms. Tim is indeed the bravest person I've ever known."  She is right: Tim is a sympathetic man who makes light of his extraordinary bravery. As he talks about the horror of the situation, he suddenly pauses and pats my arm in reassurance – he is concerned that it might be upsetting me too much.  We sit drinking tea, looking out on to his blooming garden full of roses tended by Judy.  "I am so lucky to be living in such a beautiful place. For two years afterwards I would go for a long walk every day, along the river Thames: in the sunshine, in the rain with my mackintosh. It kept 'Tim' from going mad in the house." He has a tendency to refer to himself in the third person, as though he is looking objectively upon his traumatised mind. But he has been forced to accept that the terrible images and sounds will stay with him forever. Since the September following the attack, he went through psychotherapy and cognitive behavioural therapy offered to survivors of the bombings.  He went for a counselling session as recently as yesterday and acknowledges he may never be able to work again.  Yet Tim still hopes there will be a day when he doesn't have to talk or think about "that day" any more.  What is life like for him now – the man everyone calls a hero – who for several months after the explosion could hardly walk or talk?  "I would say that I'm as recovered as I'm ever going to be," he replies matter-of-factly. There were times when he suffered frequent flashbacks, but these are becoming less common. He tried to go back to work in the December following the bomb, but, after collapsing in the school staff room, he took ill-health retirement. "That was the most negative and difficult thing for me to hear, that I was losing the opportunity to do the teaching job I loved."  Does he feel resentful towards the bomber, who has not only taken people's lives and limbs, but wreaked havoc with the mental health of so many survivors?  "I saw the bomber's body, so I can't feel angry towards him. He is dead. It would be a bit like feeling angry towards a chair or a table."  So how does he rationalise what happened? "I just can't believe that people, the bombers, would choose to spend their lives doing such things. It is a feeling of disbelief, not anger.  "There are deep mental scars," he continues. "Just like physical ones, that are never going to heal completely. I have come to accept that I won't be the person that I was, but that's fine with me."  The Miracle of Carriage 346 and The Angels of Edgware Road will be broadcast on Channel 4 tomorrow at 9pm and July 13 at 7pm
When the 7/7 bombers struck, hundreds of people on his train fled the carnage. But Tim Coulson stayed to save lives. Three years on, he is still traumatised.