The name of the subject is kind of problematic. For urban rail transit, the word metro is used in most cities around the world while subway is mainly American usage. But it would be too easy if there weren't exceptions. Transit systems in Washington and Los Angeles call themselves Metro, while Glasgow in Scotland officially has a Subway. In transcripts, subway is also used in Japan and South Korea, as American English is predominant over British English there, but metro is used in other parts of Asia. And of course do many cities or countries have their own terms, for example Underground or Tube (in London), MRT (in several Asian cities standing for mass rapid transit), U-Bahn (in Germany and Austria), T-Bane (in Sweden and Norway), Subte (in Buenos Aires).
Ambiguity also adds to the naming chaos. In America, the word metro usually is short for 'metropolitan area', while in British English, a subway is a pedestrian underpass.
Conclusion for Metrobits: since metro and subway are likewise widely in use and ambiguous, both terms will be used in a lively mixture more or less randomly throughout the website, also as a courtesy to search engine users.
How to take photos underground?
A photography permit may be required, metro companies will usually issue one for free. Some cities have banned photography in metros after 2001, but some of them have again dropped the bans later. Even New York City's MTA has completely dropped its photography ban on 23 May 2005 after massive protests of photographer associations and civil rights groups, and now explicitly permits taking photos and videos in subway stations and on trains [nppa.org], [mta.info].
Avoid using a flash. It bothers train drivers and makes photos dull.
Lights may be dim in underground stations. The use of tripods is often restricted to prevent obstruction. If not using a tripod and in case your camera cannot handle the light conditions, there's still an option: select 'burst' mode and shoot some five pictures in a row. One or two of them will probably be sharp enough for most purposes. Some stations also have furniture or ledges where a camera can rest on, either on its own or on an inconspicious pocket-sized tripod.
Despite every metro's aim is transporting as many people as possible, flocks of passengers can spoil the best architectural photo compositions. On early Sunday mornings there may be fewer passengers around (so I've been told...)
Basically, safer than road traffic. Of course, as in every place where a lot of people gather, pickpockets or robbers are likely to operate (look here for some stories). Metros in some cities are safer than in others. Generally, metros are safe, so the following suggestions are only for the paranoid:
Store your valuables in a safe place. The back pocket of your pants is not safe enough for a wallet.
In areas of a city which are known to be unsafe (like The Bronx in New York), the metro, especially the stations, should also be avoided.
The safest place to ride is in the first carriage because there is the driver (if it's not driverless).
Don't look like a tourist. Hide camera, maps, dictionary etc. from view, don't puzzle over the subway maps in trains or stations too obviously.
Driverless metros are even safer than traditional ones. They are securely monitored and none of them ever had a serious accident.
Unfortunately, subway systems are vulnerable potential targets for terrorist attacks, as are airports, shopping malls or other places with lots of people. Half a dozen serious attacks have hit different metros. The danger of terrorist attacks on metro systems persists, but probability theory indicates that chances to get involved are extremely low, considering the number of passengers (over 1 billion annually in London, for instance). Compared to driving an automobile, using a metro is a much safer option.