Their supervisor walked along the rail bed, making sure he was seen, exchanging words with his crew. No one else moved.

Along the tunnel wall were two bare wires running parallel to its length, suspended on clips at about head height. The supervisor clipped the terminals of his phone to the wires and pressed the call button.

He pressed the earpiece against the side of his head, and waited.

When he had to press the button again, there was a collective shifting of posture, of gently laying down the tools they were still holding, getting ready to move in whichever direction they had to go, and quickly.

He pressed the button for a third time, and stood, head bowed, praying for an answer.

When none came, he swiftly unclipped the phone and pointed east.

‘We’re evacuating. Green Park. Go.’

Everyone else had trained for this. Dalip had had an hour’s talk. Stanislav took a handful of Dalip’s boilersuit at the shoulder and didn’t let go.

‘With me. We walk. Watch your feet.’

‘What’s happening?’

‘We do not know.’ Stanislav half-shoved, half-dragged Dalip in the direction of Green Park. ‘That is why we are leaving. If it is nothing, we will come back and carry on. If it is something, we can find out what it is. If it is a bad thing, better we are not here, between stations.’

It started off as a low rumble which slowly reached a crescendo, and then it faded away again. Rather than Stanislav holding Dalip up, there was a mutual bracing of each other against the shaking ground.

‘Is that a bad thing?’ asked Dalip.

Stanislav gave a thin-lipped smile. ‘Not good.’

Everyone quickened their pace.

‘Does this happen often?’

‘No.’

‘Are you going to let go of me?’

‘No.’

The lights winked out, and it was utterly black for a moment. Torches were already in people’s hands, and bright blue beams cut through the dusty air like searchlights. Dalip stumbled, and was held up long enough to find his feet again on the rocky ballast. Stanislav swept his torch across the tunnel roof and floor.

‘Get yours and put it on. Put your hand through the strap and tighten it to your wrist.’

Men were walking past them in pairs, keeping an eye on each other, making sure no one was left behind.

Dalip fumbled his torch on, and fitted it around his hand as he’d been told, still with Stanislav maintaining a death-grip on his boilersuit.

‘Okay?’

Dalip nodded, realised that he couldn’t be seen, so mumbled, ‘Yes.’

‘Stay with me. Whatever happens, stay with me.’

They moved back up through the column of men and as they turned the slight corner, the green emergency lights on the platform of Green Park came into view.