I also have a Pelican M6 - very sturdy light, and it's often the main light I have in the bag I carry daily on the subway. Another plus, besides those mentioned by physics137, is that the LED drop-in module that Norad45 mentioned for his Surefire lights will also work in the Pelican M6. That makes it a very attractive option, in my opinion.

Other possible lights, with a less powerful beam than the Pelican M6 or similar Surefire light, would be either an Underwater Kinetics 4AA eLED (around 10 hours of light) or a Streamlight 4AA Propolymer 1-watt LED light - about 4 hours of light.

As a backup light, I've been carrying a CMG Infinity Ultra...if it's not a matter of smoke conditions, but merely dark, that one should be adequate, and has a very long runtime on a fresh AA battery.

And rather than chemical lights, I plan to carry 5-10 or so of the $1.25 Photon keychain light clones. Smaller and cheaper than chemlights, I've thought these might be a good option - the kind of thing that, in case of another blackout or a stuck train that needed to be evacuated, could be handed out to other passengers - more people have light, people can light obstructions for others, or lights could be duct taped to a spot to shine on an obstruction so everyone can avoid it without everyone bunching around the one or two people who might have a light (no one wants to be that popular).

In addition, I carry a water bottle (if it's even something mundane like trak work/power outage, you could be down there a long time).

Also, I carry a Leatherman Wave, hoping it could be useful (some of the NYC subway doors have Phillips-head screws holding the window panes in them).
While on the topic, anyone know ways (in an emergency only) of exiting a subway car? I know there are panels accessible by the large keys the conductor has, but are there other ways of unlocking or forcing the doors open, other than a battering ram?

Dave