Frontline (Location)

Front Line is the location of a battleground between the armies of the Fourth Reich and the Red Line. It's evident that war has been waged at that location for quite some time; shell casings litter the ground and burnt-out vehicles are numerous. Motivation for the battle is unknown, although it's probably because of contrasting ideals between the two factions, or because this area represents a significant tactical advantage to both sides.

Overview
The Front Line is located somewhere between Lubyanka station and Tverskaya station. The main fighting seems to be going on  the left platform of the tunnel. Each side has machine gun positions, field hospitals and patrols on their own territory. Soldiers of the Fourth Reich have two machine guns mounted with searchlights to monitor the area under the bridge along with several traps. This no man's land is littered with dead soldiers from both sides, a burned-out train cart, and a destroyed Panzer. Parts of the platform have been destroyed and there is a hole in the roof from where a missile penetrated the metro ceiling. Beneath this missile is an unfortunate metro inhabitant; possibly a Nazi or Communist and some military grade rounds.

In Metro: Last Light, two of the Reds in "Revolution" can be overheard discussing the events of Frontline. Elaborating that a single person (hinting towards Artyom) killed every soldier on both sides of the front. An alternate explanation given proposes that it was a supposed tunnel demon, the "Dark Rail Tracer", that wiped out the camps. The same two Reds can be heard talking about a traitor smuggling someone to the front line by hiding him in the luggage compartment in a rail car. A very active section of the front line appears again in Last Light's Faction Pack DLC, where it is the location of the Heavy Squad mission.

Trivia

 * The Front Line may be completely deserted if the player decided to kill all inhabitants.
 * Correspondingly there may be no change whatsoever to the front line, or even a tactical sway from one side to the other.
 * As a side note in Metro 2033 Redux, if you completely wipe out the Nazis, but sneak past the Reds, occasionally Red soldiers will wonder across No Man's Land to Reich territory. If they don't spot you, they return back to their side without taking advantage of the situation.
 * The Front Line seems to be in an endless stalemate. Red soldiers complain about their commanders refusing to change tactics, simply just attacking head on hoping that the Nazi defenses will eventually break, instead of trying to outflank them and bring a swift end to the battle. It's implied that the Front Line remains in a stalemate even after Artyom killed everyone on both sides.
 * The front line between the Reds and the Fascists takes place on the Tagansko-Krasnopresnenskaya line, somewhere between Kuznetsky Most and Pushkinskaya stations.
 * The Nazis side of the Front Line seems to be much better defended. They have all three levels of the bridge covered with machine guns, spotlights, and tripwires. The Red Line only seems to have the tracks covered. The Nazis also seem to have multiple fall back positions. This might just be for gameplay challenge or more proof of the Nazis superior military planning and tactics.
 * There is a lower access that allows someone to go on each end of Front Line without being shot from both sides, but no one seems to use it except Artyom. The Nazi side has several Grenade tripwire placed around it however.
 * In Metro 2033 Redux, the remains of a long-dead Reich recon team can be found underneath the Red side of the bridge, huddled around their still-burning campfire. Curiously enough, they didn't equip their gas masks despite being in a toxic area, making their cause of death rather unsettling.