Salvaged from an unknown tunnel or station and built up with a massive amount of steel, the armored train stands as the single most dangerous weapon of war in possession of the Red Line. Despite this, it remains relatively inactive and unheard of for most of Metro Last Light. It is a set-piece item for the opening of the level Revolution, where it is seen being constructed, and is mentioned in passing by Andrew in Regina but is not used and not vital to the plot until the final wave of enemies in D6. The armored train is possibly the answer to the Rangers' fortification of D6 and the "Panzers" used by the Nazis. In its brief last appearance, it is the deciding weapon that cripples the Rangers by destroying D6's platform - who had previously killed several dozen soldiers of the Red Army, a tank, as well as a shield and flamethrower unit. Though it is obviously destroyed in the C'est la Vieending to Last Light, what exactly happens to it in the Redemption ending is unknown, though it is possible that the train was rendered useless by destroying the platform.
A very similar armored train is seen again in Metro Exodus, this time operated by Hanza. It intercepts the Aurora as Aryom and the Rangers are escaping Moscow. Artyom is forced to sneak aboard the train and plant an explosive in the boiler. He then jumps back aboard the Aurora and blows it up.
In the level Separation, Reich has an armored train which can be seen in most of the entire level, with their logo on the front replacing the red star.
It is unknown what type of engine and fuel the original Red Line armored train uses. The base of the train are standard Metro carriages, which are electrically-powered, but as electric 3rd rails in the Metro tunnels aren't powered anymore, it can't be using their original propulsion. Furthermore, the train might be too heavy to be powered by the original set of electromotors while still maintaining the great speed seen in Metro Last Light, given all its additionally-mounted armor. It is possible (and likely) the Reds added a combustion engine (salvaged from one of the many locomotives or armored vehicles found on the surface) or a steam engine to power the train.
The armored train used by Hanza in Metro Exodus is steam-powered, as it has a firebox very similar to that found on the Aurora and also a chimney producing a lot of steam and smoke on the top. The original Red Line Train doesn't have a chimney though, so it's unlikely to be steam-powered.