Armored trains had been in use in the Soviet Union since the Bolshevik Revolution and many were in use when the Germans invaded in June 1941.  Many of these trains were found to be large, obsolete, and easy to destroy.  They also mounted two heavy guns in a single artillery car, which when destroyed would deprive the train of half its firepower.

In late October 1941 the State Committee of Defense (GKO) ordered that new improvements be made in armored train design.  The new train was to have a completely new artillery car mounting only one heavy gun.  The car was also smaller than the artillery car of earlier trains so that it would present a smaller target silhouette to enemy gunners.  This also meant that the loss of one car would not reduce the train's total firepower as much as the large two gun cars did.

Work was carried out very quickly and plans were delivered to a variety of factories as soon as they were completed.  Orders were received by some 43 rail yards and 9 NKPS factories.  Not surprisingly this led to problems in standardization as each factory had different production capabilities and could only use local resources.  This led to only four trains being equipped with the hardened steel armor originally intended for them, the rest were fitted with two thin sheets of steel with a layer of concrete poured between them.

Armor was not the only inconsistency.  Modern 76mm artillery pieces for the new trains were in short supply as many had been sent to the front or used in T-34 tanks.  Because of this the trains were forced to use whatever 76mm gun they had on hand at the time.  This led to interesting mixes of older gun systems taken from old M-1902 field guns, M-1927 field guns, M-1914 anti-aircraft guns, and even L-10 tank guns.  Some guns lacked optics and few had spare parts on hand for repairs.  Only later were they able to get the modern F-22 and F-34 guns for their turrets.  

Machine gun armament suffered from the same availability problems as the heavy guns.  Maxim 7.62mm machine guns were the preferred weapon because they could lay down sustained fire without overheating, but Degtyrev DT machine guns were also used.  In some cases the Soviets employed Browning Automatic Rifles which they had captured from the Poles a few years earlier.

The locomotive was typically of the OV or OK series that was covered with steel plates.  It was equipped with a DShK machine gun in an open position for air defense.  A radio was located in the locomotive for communications with other units.  Internal communication was made possible by a system of speaking tubes, and light and tone signals.  This system linked the engineers with the commanders of the artillery cars.  Another interesting feature of the locomotive was that it had a steam heating system that was linked to all of its tender and artillery cars.  

The tender car (which was not always used) was armored in a similar manner as the locomotive, and mounted four Maxim machine guns in an anti-aircraft mount for air defense.

Artillery cars of the OB-3 had a crew of 12 men and were distinguished by their hexagonal turret which mounted the 76mm main gun.  Two machine guns were placed on each side of the car to provide anti-personnel fire.  No optics were fitted to the car so all observations had to be accomplished by looking through vision slits or by having a crewmember open a hatch and look outside.  Hatches in the bottom of the car allowed the crew to escape under combat conditions. 

The PVO-4 anti-aircraft cars were simple flat cars that had two 37mm M-1939 anti-aircraft guns placed on them.  These were placed at the front and rear of the car and surrounded by a box of thin steel panels as armor.  The panels were kept up and the guns faced the direction of travel when the train was in route, but the panels were folded down when the guns engaged aircraft.

Security cars completed the train and were simple flatcars attached to either end.  They carried tools, spare ties, connectors, and track sections for repairing damaged sections of track.  Often they would be manned by a small infantry unit who used sandbags and railroad ties for cover.  These troops would act as observers and often dismount if the train was attacked.

The first combat use of the OB-3 trains came in 1942 where they saw action in Voronezh, Kharkov, Stalingrad, and the Northern Caucasus region.  Some were used to cover the retreat of units attempting to regroup, while other trains were used as assault elements to blunt German armored attacks and bring heavy artillery were it was needed.

Most of the 20 OB-3 trains lost over the course of the war were destroyed in desperate combat actions of 1942.  In many cases the crews fought against hopeless odds (often under attack by both aviation and armor) simply to inflict as much damage on the enemy before they themselves were destroyed.

While it is known that the OB-3 trains fought until the end of the war, their eventual fate is not known.  Soviet sources say little about the post-war fate of the armored trains.  Some few ended up in war museums and the remainder were most likely scrapped.

When the OB-3 was deployed in 1942 they were organized into armored train battalions usually consisting of two trains.  In combat they were used as assault elements to stop German armored attacks.  The crews fought long difficult battles against combined air and ground assaults, but often refused to retreat.  Some trains were completely destroyed as a result of these heroic last stands.  Additionally, some OB-3's were even used as anti-aircraft platforms - which was a task that they were not designed for.

Like any armored train the OB-3 had many advantages.  It could bring large amounts of artillery to bear on a target, carry large amounts of ammunition, and move into an area quickly - often by surprise.  An armored train could also cover 500 kilometers of ground each day.  Its armor was heavy an rendered it immune to small arms and shrapnel, and it also carried its own anti-aircraft armament.  Its many machine guns also enabled it to provide friendly infantry with cover fire.  They also could be used for transport and in some cases the trains helped carry infantry units to the front.

In addition to these advantages the OB-3 armored train was much lighter and easier to produce than the earlier BP-35 and NKPS-42 trains.  Its two axle artillery cars were smaller than earlier types and only mounted one gun, which meant that if one car was destroyed the train would only lose one gun.  In earlier models the loss of an artillery car meant that the train lost half of its anti-tank firepower.

Unfortunately the OB-3 armored train suffered from a number of disadvantages.  The first was typical of any armored train whose maneuverability was limited by its tracks.  In many cases armored trains were trapped when the tracks ahead of them and behind were destroyed.  They were also exceptionally vulnerable to air attacks.  The trains were also very dependant on their bases for support as water and coal had to be taken on daily.  As the war progressed they also proved to be less effective than tanks when it came to engaging other armor units.

The lack of standardization and spare parts only complicated an already weak Soviet logistics situation.  The low quality armor resulted in heavy crew losses.