Normally the gas follows the bullet out of the muzzle and sets fire to the oxygen in the air close by. Hence muzzle flash. But with a hard contact shot to the head like passenger number four had chosen, the bullet makes a hole in the skin and the gas pumps itself straight in after it. It expands violently under the skin and either rips itself a huge star-shaped exit wound, or it blows all the flesh and skin right off the bone and unwraps the skull completely, like peeling a banana upside down.

That was what had happened in this case. The woman’s face was reduced to rags and tatters of bloody flesh hanging off shattered bone. The bullet had travelled vertically through her mouth and had dumped its massive kinetic energy in her brain pan, and the sudden huge pressure had sought relief and found it where the plates of her skull had sealed themselves way back in childhood. They had burst open again and the pressure had pasted three or four large fragments of bone all over the wall above and behind her. One way or the other her head was basically gone. But the graffiti-resistant fibreglass was doing its job. White bone and dark blood and grey tissue were running down the slick surface, not sticking, leaving thin snail trails behind. The woman’s body had collapsed into a slumped position on the bench. Her right index finger was still hooked through the trigger guard. The gun had bounced off her thigh and was resting on the seat next to her.

The sound of the shot was still ringing in my ears. Behind me I could hear muted sounds. I could smell the woman’s blood. I ducked forward and checked her bag. Empty. I unzipped her jacket and opened it up. Nothing there, just a white cotton blouse and the stink of voided bowel and bladder.

I found the emergency panel and called through to the conductor myself. I said, ‘Suicide by gunshot. Last but one car. It’s all over now. We’re secure. No further threat.’ I didn’t want to wait until the NYPD assembled SWAT teams and body armour and rifles and came in all stealthy. That could take a long lime.