When I got on the train, I saw a binbag near the doors (which appeared to be containing a pedal bin from the shape of it), although nobody was sitting next to it. There was a person sitting nearby, so I thought that it might be his (if he put it where there was more space), or it might be abandoned. I kept an eye on people's movements, and when my "prime candidate" was getting off the train I saw that there was a rucksack left on the floor where he'd been sitting. I said "Excuse me, I think you've forgotten your bag", but he said that it wasn't his. I asked if anyone else claimed it, and there was a general head-shaking/muttered "no"s from the other passengers. "How about that bin bag?" Again, no. Hmm.
So, on the one hand my duty seemed clear - I should report this. But given that the train was in motion, the only way I could see to do that was to hit the emergency alarm. I've never used that (or seen it used), so I don't know whether you can then talk to the driver, or whether it just brings the train to a screeching halt in the tunnel. And it didn't really feel like an emergency situation. Also, I was running late for work already (as usual), and I didn't want any extra delay. I decided that I'd wait for my stop (another two stations down the line), and as long as the bags didn't explode by then, I'd report it to someone when I got off the train.
When I got to the station (Embankment), I had a look for someone on the platform, since they often have a member of staff signalling when everyone is clear of the doors. However, in this case, there was nobody around. Plan B - go to the front of the train and talk to the driver directly. Unfortunately, I was in the rearmost carriage, and I didn't have time to hurry to the front of the train before it had started pulling out of the platform. Plan C - use the intercom on the platform. They had buttons for Emergency and Information. Again, it didn't feel like an emergency, so I used the information one (although I was giving it rather than receiving it). I told the person what had happened, and to his credit he sounded a lot less bored once I'd given him the information (rearmost carriage, eastbound train). So, it worked out ok - no "boom" while I was around, and I delegated it to other people after that.
Generally speaking, I'm aware that Al Qaeda are responsible for a lot of deaths, but when it comes to bombs in London I'm more concerned about the IRA (I was working in the Docklands 8 years ago when they broke the ceasefire, and it was blind luck that I'd left the office in time). And I'm not exactly living in terror of the IRA either, anymore than I'm afraid that every stranger I see on the street is a potential mugger. So, this is more about civic responsibility, since my actions (or inactions) could affect other people. I've filled out a feedback form on the TFL site, so I'll see whether they offer any useful advice. (Edit: They said that I did the right thing, and that I should feel free to use the emergency cord if it happens again in the future.)
So, poll time...
If you saw an unattended bag near you, would you...?
Hit the emergency alarm right away
Change carriage/train at the next stop, but say nothing
Get off the train at the next stop, and tell someone
Get off the train at your stop, and tell someone
Ignore it completely
If you were travelling, and your journey was delayed due to me reporting an unattended bag, would you think...?
Well done that man - better safe than sorry
Git - now I'm going to be late!
Meh - I've got a book, so I'm happy