I dismissed the thought because of tactical implausibility. The timing was wrong. The New York subway would make a fine target for a suicide bombing. The 6 train would be as good as any other and better than most. It stops under Grand Central Terminal. Eight in the morning, six at night, a crowded car, forty seated, 148 standing, wait until the doors open on packed platforms, push the button. A hundred dead, a couple of hundred grievously injured, panic, infrastructure damage, possibly fire, a major transportation hub shut down for days or weeks and maybe never really trusted again. A significant score, for people whose heads work in ways we can’t quite understand.

But not at two o’clock in the morning.

Not in a car holding just six people. Not when Grand Central’s subway platforms would hold only drifting trash and empty cups and a couple of old homeless guys on benches.

The train stopped at Astor Place. The doors hissed open. No one got on. No one got off. The doors thumped shut again and the motors whined and the train moved on.

The bullet points stayed lit up.

The first was the obvious no-brainer: inappropriate clothing. By now explosive belts are as evolved as baseball gloves. Take a three-foot by two-foot sheet of heavy canvas, fold once longitudinally, and you have a continuous pocket a foot deep. Wrap the pocket around the bomber, and sew it together in back. Zippers or snaps can lead to second thoughts. Insert a stockade of dynamite sticks into the pocket all the way around, wire them up, pack nails or ball bearings into the voids, sew the top seam shut, add crude shoulder straps to take the weight. Altogether effective, but altogether bulky. The only practical concealment, an oversized garment like a padded winter parka. Never appropriate in the Middle East, and plausible in New York maybe three months in twelve.

But this was September, and it was as hot as summer, and ten degrees hotter underground. I was wearing a T-shirt. Passenger number four was wearing a North Face down jacket, black, puffy, shiny, a little too large and zipped to her chin.