The reason for its abandonment was clearly the change within the methods of warfare, as tanks and motorized infantry dictated military doctrines that slowly pushed out the armored train. Since trains were limited to railroads, they were more vulnerable to bombers and artillery.
In addition to that, the railways were more and more subjected to acts of sabotage by commando or partisan units, which slowed the advance of the trains significantly. The mere fact that it relied on the use of tracks turned these war machines into vulnerable giants.
Nevertheless, trains continued to serve in battle even after WWII (but far less actively), most notably in Indochina.
But in the countries of the Eastern Bloc, the use of trains as means of battle was nurtured as a tradition. Even though it was old-fashioned in a way, it was still suitable for serving as a mobile intercontinental ballistic missile (ICBM) launching ramp. In the late stages of the Cold War, the RT-23 Molodets, an intercontinental ballistic missile, entered service in the Soviet Union during the Cold War.
It was capable of carrying a nuclear warhead. One of the options for transporting and launching the missile was from a specially designed train across the Trans-Siberian Railway. The strategic importance of this railway was emphasized during the 1970s after the split between the Soviet and the Chinese government. According to different accounts, four or five armored trains were built in order to protect the southeastern borders of USSR.
Every train included ten main battle tanks, two light amphibious tanks, several AA guns, as well as several armored personnel carriers, supply vehicles and equipment for railway repairs. They were all mounted on open platforms or in special rail cars. Different parts of the train were protected with 5–20mm-thick armor.
So it is not surprising that some of the last known uses of armored trains happened during the conflicts following the collapse of the Soviet Union 1990s, most notably in the disputed area of Nagorno-Karabakh, between today’s Armenia and Azerbaijan.
Also, during the Yugoslav Wars from 1991 to 2001, some improvised armored trains were used by paramilitaries in the conflict in Croatia and Bosnia. These were regular passenger trains transformed into terrible land cruisers, capable of laying siege to towns and villages across the war-torn Bosnia.
The most infamous train that was in service during those years was the Krajina Ekspres, employed by the members of a Serbian paramilitary in Bosnia. The train took part in a three-year-long siege of the town of Bihac, which lasted from 1992 to 1995.
Even then the technology was considered to be obsolete, but in a conflict between various paramilitary and guerilla groups, such hardware proved to be intimidating. In late 2015, Pro-Russian militants in the Donbass region of Ukraine were pictured operating a homemade armored train.
One armored train that remains in regular use is that of Kim Il-sung and Kim Jong-il, which the former received as a gift from the Soviet Union and the latter used heavily for state visits to China and Russia as he had a fear of flying.