We were around the curve and out of sight of the platform when we reached the place where the blood trail started. So far the ballast and dirt of the track bed had been pretty efficient at soaking up the blood, but here my torch flashed on a sleek irregular pool of dark red.

‘I’m going to check further up the tracks – see if I can find where he got in,’ said Kumar. ‘Will you be all right here?’

‘Don’t worry about me,’ I said. ‘I’m good.’

I crouched down and methodically quartered the area around the pool of blood with the beam of my torch. Less than half a metre back towards the platform I found a brown leather oblong and my torch reflected off the shiny face of a dead or deactivated phone. I almost picked it up but stopped myself.

I was wearing gloves and had a pocket full of evidence bags and labels and had this been an assault or a burglary or any other lesser offence I’d have bagged and tagged it myself. But this was a murder inquiry and woe betide any officer who breaks the chain of evidence, for they will be sat down and have what went wrong with the OJ Simpson murder trial explained to them at great length. With Power-Point slides.

I pulled my airwave set out of my pocket, fumbled the batteries back in, called the Exhibits Officer and told him that I had some exhibits for him. I was double-checking the area while I waited when I noticed something odd about the pool of blood. Blood is thicker than water, especially when it’s started to congeal, and so a pool of it doesn’t flatten out in the same way. And, I noticed, it can obscure the thing it’s covering. I leaned in as close as I could without risking contaminating it with my breath. As I did I got a flash of heat, coal dust and an eye-watering shit smell that was like falling face down in a farmyard. I actually sneezed. Now that was vestigia.

I went down on my front to see if I could work out what it was under all that blood. It was triangular and biscuit-coloured. I thought it was a stone at first but I saw the edged were sharp and realised it was a shard of pottery.

‘Something else?’ asked a voice above me – a forensics tech.

I pointed out the things I’d found and then got out of the way as the photographer stepped in to record them in situ. I shone my torch up the tunnel and caught a reflection off Kumar’s high-visibility jacket thirty metres further on. He flashed back and I walked, carefully, up to join him.

‘Anything?’ I asked.