I'm thinking of a scenario similar to that of the Metro 2033 series (minus the magic, paranormal and mutants)
A worst case scenario nuclear apocalypse has happened: 90% of the world's nuclear arsenal (including dirty bombs) has been detonated over most metropolitan areas in the world. Electronic equipment has been EMP'd, communication lines have been severed and governments/militaries have been decimated. The immediate survivors are those who were underground (subway, tunnels, sewers, catacombs, etc.) at the moment of the detonations as well as those away from cities.
The wind soon picks up soot, smoke, and radioactive fallout and spreads it through the world, drastically diminishing the survival rate of those who survived in the country.
In City X, which has a vast underground (London, Moscow, New York, Paris, Tokyo?, Beijing?) there is a large group of survivors (5,000+) spread through the tunnels. Will they be able to survive there until the surface becomes habitable? How would they go about it?
Assume no outside help, current technology and the least possible amount of outlandish situations (there's no livestock underground unless it's absolutely necessary for the survival of the people trapped there)