Metro Prequel

One year ago, the world erupted in flame, terror, desolation and radiation. World War III only lasted for a few minutes, but it decimated civilization and rendered the majority of earth uninhabitable. A small fraction of Moscow's population scrapes by living a new sort of life underground. In the dilapidated ruins of Moscow's sprawling metro system, life is cold, dark, and perilous. You are a contractor. Your knowledge of the tunnels and skills in combat are in high demand. This job is beginning like any other, guide a few refugees from one metro station to another. Three women, and a little boy named Artyom. They're your clients, your cargo. Do not let your guard down, do not investigate strange noises, do not take unnecessary risks. If something goes wrong, there are no second chances down here. It's cold, it's dark, and you are alone. -- This was a request! Luckily, there was a bit in the wiki about Artyom's mother that said she traveled from Botanichesky Sad to Timiryazevskaya Station shortly after the war, and that was it. No other context given. So here's the context! (Unofficial and noncanon, but still plausible, right?)