There are a few things everything needs to live. Food and shelter are high up the list.
The metros have a surprising amount of space in them, for those unaccustomed to subway systems. Even the smallest station is 2-3 tracks wide, with a platform the length of the train, and wide enough to safely accomodate the flow of traffic. The largest of stations include the bottom level of malls, large buildings (libraries, theaters, etc), or rival large airports in scope and scale. Still, while there's more room than one might expect, quarters are cramped at best. An entire complex for the fabulously wealthy of the Metro is still likely smaller than the bathroom of the ultra rich before the calamity.
Stations always have a few basic features, and can be grouped into several archetypes. Stations always have at least one lower level with tracks, with at least one track going either direction, and often a third "express" track. There will always be at least -a basic platform to service these tracks, and this area is typically the full length of a subway train. Above this, there's a floor with turnstiles, ticket booth, maps, and perhaps a vendor or two, which is generally but not necessarily smaller than the platform below it. Stations of substantial size may have a multiple platform levels to accomodate large volumes of traffic. From there, the stations gradually increase in size. More space on lower platforms, more rails, and the upper levels may have a few small shops built into them. From there, you get into the larger stations. Large stations fall into intermodal stations, malls, and those built into the sublevels of things like large hotels, major arenas, libraries, and other large public buildings. Intermodal stations and other stations with large, above ground parking structures or other such buildings that may serve as usable (or potentially reclaimable, for the daring stalker) space. All medium large to large stations will at least have several built in shops and kiosks, which are generally repurposed.
Regardless of size, most city-stations will wind up with a fairly standard setup. Entrances and exits, both to the tunnels and to the surface, will be closed off with gates or doors. Most surface exits will often be sealed semi-permantenly save a few well placed, defendable gates. Exits into the rail system will be possible to open and close to permit traffic of various sorts, both on foot and by various carts made by recycling the old trains. Housing typically is built from scraps and material taken from the trains. Most houses are about the same size, and generally are shared by multiple inhabitants. Bunk beds and lofts are exceptionally common, even for those wealthy enough to afford a whole room to themselves. On average, a room is around 6-8' wide by 10-12' long, although height varies slightly. Smaller and larger rooms are sometimes found, but total space per person remains roughly consistant. Bathrooms are handle dormatory/barracks style, with shared communal spaces. Water availability varies by location, with camp showers or sponge baths being the norm. Food is also generally stored communally, with a reclaimed walk in fridge/freezer if possible, with standard metal lockers mounted inside for rental. Rooms are generally stacked at least 2 high, and up to 3 or 4 where ceiling height, materials, and builder capability allow.
Dead broke level living is squatting in a tunnel. There's shelter from the elements, and the outer tunnels near citystation are generally somewhat secure, but there's nothing resembling any amenities.
Poor level gets you an emergency bunk from the initial rush to the Metros, small enough to make Naval bunks feel incredibly spacious. You might even have a footlocker.
Struggling level gets you 1/4 of a room, generally managed by way of bunk beds, bunked lofts if the room is tall enough, or two bunked beds large enough for two. You'll generally have a footlocker and/or small dresser, and a camp stove. Usually a bench or couch, or several small (possibly folding) chairs.
Average is two to a room. Either a single set of bunks, a bed for two, or two lofts. There may be a designated kitchen area, with a freestanding kitchen assembly, and possibly even a small icebox.
Wealthy is a whole room to yourself. Lofts are still favored, but you have a LOT of room compared to most.
Very wealthy is two rooms. Setups vary, but by virtue of the design of the Metros, you generally have to either get one atop the other, or two in a row. These rooms may or may not be connected directly.
Filthy Rich and beyond are largely unavailable. Examples might be an eccentric billionaire who built a sprawling subterranean farm for no apparent reason, and now is the sole provider of high quality fruits, a savvy survivalist who saved up the skills and supplies needed to get ahead, or someone who found a tremendous stash of pre-calamity supplies and information, or even someplace that could be converted into an entire new town.
Most towns will have the usual variety of a market/bazaar, pub, some manner of town hall to handle management duties, and an area set aside for work. Those that have skylights and the like will usually have some manner of hanging garden.
Some stations are new, having been excavated to make more space, a building basement tunneled into, sealed, and reclaimed, a discovered cave, or whathaveyou. Lacking the solid concrete, these tend to have more of a frontier town kind of feel, and are less safe from attack, with more acceptance of the carriage of major arms for defense as a result. There's a great deal more freedom, often more space, and a chance at fortunes, enough for many to brave the risks.
Thanks to the farming habits of several primarily subterranean races, food is slightly easier than one might expect living underground. Deep Taters, developed by several races, are similar to potatoes, but instead grow their extremely durable roots deep, hanging the tubers much in the manner of fruit on a tree, and form the staple of most diets. A number of edible mushrooms have similarly been introduced and bred. Various forms of aquatic life flourish in cavern pools that sometimes connect to the Metro system, either accidentally or deliberately made and stocked. However mutated, much of the meat of post cataclysm animals remains edible. Various types of farms exist, using light collecting skylights or upper levels of Metro stations with durable windows, along with some above ground growing and harvesting (old parking structures and some chain link fencing make a good enclosure for raising livestock and window gardens). In some places, grow lights are used, generally powered by manually cranked/pedaled generators unless good power generation is available.
Apocalypse or not, life goes on, and with it trade. Barter is common, but with the difficulties of travel and time since the fall, a new currency has been developed to make trade a bit easier. Ammunition and medicine are common barter goods that are readily transported, along with luxuries such as drugs and alcohol (high quality alcohol and things like cigarettes or equivalent). Currency is handled by the Trader's Guild, and tends to be very durable and quiet (coins have leather wraps to keep them from making noise, etc.) Money is minted at the main hall of the Merchant's Guild, or occasionally by representatives in larger stations that are determined to have their own currencies.
In the years since the Calamity, a lot has changed on the surface. Cities crumble, wildlife mutates and moves back into the abandoned sprawl. Access to the surface runs the gamut, from irradiated mutated hellscape, to "A bit rough on the lungs without a gas mask after a while". Most stations with sufficient manpower and material will move to secure a stronghold on the surface around the station exit to permit safer travel to the surface. A few surface routes have been cleared and mostly secured, between stations that are nearby but lacking in subterrainean connections (collapsed tunnels, no connecting lines, etc). These surface paths tend to be traveled only by well armed groups, although those wishing to explore often do so from these approximately safe paths. Running into other people on the surface can be tense, but most bandits stick to the tunnels.
When the calamity happened, the cities basically went underground, and the people took their trades with them. Some jobs that were no longer useful or possible went away, and others flourished. Farming is a fairly common occupation, as is working the manual generators needed to power grow-lights for places without access to surface sunlight. In areas with relatively safe surfaces, some semi-domesticated wildlife and various hearty crops can be cultivated. Leatherworking is common, to make up for the loss of access to many traditional space-intensive crop-based fabrics, or their specialty material intensive synthetic counterparts (although finding and/or repairing pre-calamity textiles for use is lucrative). Entertainment is important to keep morale up, and all manner of musicians, storytellers, and the like can be found. Travel is still something of an ordeal, so mail carriers, deliverypeople, traders, and anyone else who travels between stations outside of the main line tend to be brave and handy with a firearm. Most jobs related to copious regulatory compliance disappeared in the Calamity, although a few stations have decided that red tape is the key to survival.
Space is limited, but downtime is fairly plentiful. Reading, music, and art are all popular. Bars, pubs, and the like often will have darts, or the more common Metro equivalent of Knives (dart rules, played with throwing knives. In larger stations they'll generally be Tiny Throwing Knives ($15, -1 damage version of small throwing knife) or smaller Shuriken. Frontier stations will generally have a bit more space, and a more casual attitude towards weaponry and allow the use of Small or even Large throwing knives, or larger shuriken. Various martial arts are commonly taught and practiced, and enjoyed as spectator sports. There are several races a year performed by those who have their own carts specialized for speed (often various merchants and delivery people who have their own racecarts and know the rails), and generally attended by anyone in the stations they pass through and everyone who can travel safely to get there (safety in numbers, number of travelling spectators makes it a safer trip).
Luxury makes life a little more bearable when you can barely even see the sun, and feeling the breeze on your face means you are likely to breathe in a big dose of toxic crap. Alcohol is a favorite, having existed for about as long as sapient races have. In the Metro, it's usually derived from deep taters and other such crops as a distilled beverage. Alcohol is taken neat, with mixers and beverages such as beer or wine being luxuries, due to the ingredients required and space taken to store a single drink. Beverages such as Apple Pie Moonshine are greatly enjoyed, and quite lucrative, offering variety and flavor in a very strong package. Tobacco or equivalent is available, with suitable stand-ins found among plants and mushrooms. Music is common, most often played live, although recovered or new production players with pre-calamity songs are greatly prized. Recovered projectors, movies, televisions, etc. are worth a fortune, and someone able to recover one with a stash of old shows or movies able to set up a small theater quite easily.
Medicine and medical care are a great part of modern life, and will be discussed in Metro Medicine, along with a variety of drugs and treatments.
Getting around in the Metros is something of a process. Everything in a given Metro is within easy walking distance, but getting from one to another can be a challenge. Main lines will tend to have an armored train with several cars, and ample guns and gun ports, along with a fair bit of storage. The main car is generally powered, with additional power available from a handcar lever. In addition, anyone with compact railcars (small enough to be removed from the tracks easily) may use the rails, as well as pedestrians along access paths. Main railways tend to be reasonably safe by virtue of quantity of travelers. Travel on smaller rails is a bit trickier. There might be a railbus drasine that runs a regular route, but it's nowhere near as large or well defended as the main rail's armored train(s). The smallest routes are likely to completely lack anything resembling a regular transport system. If you want to ride the rails, you have to own or hire a railcar, and find some people looking to travel to provide extra guns.
Usually delivery people will have some manner of railcar, often HEAVILY customized. Some have little to no armor, and are basically a luge designed to simply go too fast for trouble to catch, others are tiny tanks that trundle along, and anywhere in between.
Foot traffic is the old standby to get around, slow but fairly reliable. Bicycles (both regular and converted for rail compatibility) are also fairly common, particularly where they could be salvaged off the surface (they're also one of the only fairly available ways to get around on the surface at speed).
Offroad trucks and stuff exist, but they're generally old and patched together, because the manufacturing capability to build new full sized engines is mostly gone, so engines require a LOT of manual labor to produce by hand.
All manner of critters could be found in and around the Wastelands even before the Calamity, with rampant mutations and wider availability of horrific toxins, chemicals, and such only permitting greater variety of natural armaments. Prey animals tend to be large and powerful, and/or have powerful natural weaponry such as innate flamethrowers, sonic attacks, or launchable spines. Predators are even larger and more powerful to compensate, hunt in packs, or both. That said, a number of species split, some maintaining their present size and lifestyle, other developing into new species. A few examples are below.
Diggers: a broad category of mutated omnivores derived from ground dwelling species. Rats, moles, voles, etc. They wound up semi-upright, with even larger claws and teeth. Dangerous in large groups, some of them can be quite tame if raised by humans to be so. A lot of factors determine if they're a threat or not, but it's typically pretty apparent which you're dealing with. They tend to hang out in tunnels, basements, and low areas on the surface.
Firepigs: Somewhere between a wild boar, a goat, and a flamethrower, they've got big horns, big tusks, and often bad attitudes. One of the newer domesticated animals. Smaller breeds make fairly good pets, larger ones are generally raised around the edges of a station, or on the surface. Trained to fight and armored, they'll go tusk to tusk with anything in the Metros and hold their own pretty darn well. They produce both milk and fuel, and aid in the disposal of garbage. They generally aren't raised for slaughter due to their other uses, although wild ones are hunted on occasionally. They have no particular preference of habitat. Mostly preferring vegetation, they're still omnivores and will scavenge carcasses or eat the corpses of predators they successfully slay.
MurderBirds: Basically velociraptors with beaks. Big, mean pack hunters, with dangerous temperments and mean streaks a mile wide. Resilient and wily, they cause problems when they move in. A few breeds have learned to co-exist with the sapient races, but most are too ornery and have to be put down to ensure anything like safety. They prefer the surface, and can kind of glide if they find a high place to launch from. Tastes like a 200 lb turkey.