Updated info:
Here you go, a nice story post about this wagon of the armoured train BePo-52/BePo-662 (according to the link), where it was repurposed by the axis as a track guarding train:
https://kv-bear.livejournal.com/458008.html
Parts of the conversation in the comments of the link appear to talk about the BePo-51 and BePo-52 that were destroyed in September 1941. By looking at the pics of the damaged wagons, some work was done between the photos to remove the lower armour and make it ready to be reseated on the rails.
The following is according to the Tankfront website, though some details are inconsistent, but here’s a Google translation I’ve retouched slightly to suit a document on armoured trains I’ve been tinkering with along with one on the Polish army):
At the end of November 1941 the 6th ODBP received new equipment manufactured in Moscow, as the old ones had been destroyed in the battle around the Makoshino bridge in September.
Construction of the artillery wagons for these two trains was done by placing tank hulls (assuming without the running gear) on the base flat wagon, and then sheeting over the hulls with the outer frame, a situation which caused issues with the crew making good use of their working space, and explains why the machine guns on the sides are located where they are. The guns in all turrets were badly worn out.
Armored Train No. 1 “Fascism Fighter” «Истребитель фашизма» - BePo-51
Built at the Moscow-Passenger Depot of the Lenin and Stalin railway workshops (Google miss translate that I haven’t sorted), and consisted of an armored steam locomotive Ov No.4748 with an air defense tower (12-15mm non hardened armor) and two artillery wagons - the first a 2 axle flat car with two 45 mm guns (guns replaced December 1941) in T26 tank turrets (side thickness 45 mm) and the second a 2 axle flat car with turrets from the T-28 tank (machine gun and gun) and side thickness 17 mm.
For anti air defence, the division’s workers built 2 wagons in October/November 1941. The first was a 2 axle flat car with a central casement made of 24mm armour for a 37mm AA gun. The second was a four axle AA wagon with a 76mmL55 M1931 AA gun in a central casement of 18-30mm armour, plus Maxim HMGs (either a dual or quad mount?) and a 45mmL46 pedestal mount AA gun (21-K) both fitted at either end of the wagon.
In the spring of 1942, another 2 axle AA wagon was built by the division’s workers, armed with a quad Maxim mount and a 37mmL67 M1939 (61-K) AA gun, most likely in a standard 2 casement layout.
Armoured Train Number 2 “Let’s Destroy the Enemy.” «Уничтожим врага» - BePo-52
Built at the Lyublino and Ilyicha depots of the Western Railway. It included an armored locomotive Ov with an air defense turret (non-hardened 30-40mm armor), a four-axle armored platform (30mm armor) with turrets from a T-28 tank (guns were replaced June 1942), and a two-axle armored platform (30 mm armor) with two T-26 tanks with 45 mm guns (guns were replaced in December 1941).
For anti air defence, the division’s workers built 2 wagons in October/November 1941. The first was a 2 axle flat car with a casement made of 24mm armour which housed a 37mm AA gun. The second was a four axle AA wagon with a 76mmL55 M1931 AA gun in a central casement of 18-30mm armour, plus Maxim HMGs (either a dual or quad mount?) fitted at one end of the wagon.
In the spring of 1942, another 2 axle AA wagon was built by the division’s workers, armed with two DShK in a standard 2 casement layout.
Operational Use:
From the first days of December 1941, the 6th ODBP operated jointly with the 331st Rifle Division of the 20th Army of the Western Front in the Khlebnikovo - Lobnya - Krasnaya Polyana area, from January 21 to February 23, 1942 it was in reserve, and then until the beginning of 1943 - as part of the 33rd army, supporting the actions of the 53rd, 160th and 222nd rifle divisions in the area of the Ugryumovo junction.
On March 31, 1942, supporting units of the 277th Infantry Division, Bepo-51 derailed due to railway track being broken by German artillery. As a result, the armored steam locomotive OV No. 485 and four artillery armored platforms were damaged. After reinstatement, the whole train was sent to Kaluga for repairs.
Based on the order of the head of the GBTU KA No. 1109420, dated 5th December 1942, the armored trains received new numbers: BePo-51 became BePo-646, and BePo-52 became BePo-662.
From January 1 to March 1, 1943, the division supported units of the 33rd Army advancing in the direction of Vyazma, destroying two houses with German soldiers, two dugouts, three bunkers, suppressed the fire of three artillery and mortar batteries, and scattered up to a company of infantry,
From the 1st of March, the division was placed at the disposal of the 50th Army.
In the spring of 1943, BePo-646 was replaced with new equipment of the BP-43 type. It consisted of the armored steam locomotive Op No. 485, artillery armored platforms No. 42-629, 42-630, 42-631 and 42-632, air defense platform-4 (I assume the original author intended 4 platforms).
In December 1943, the 6th ODBP, part of the 49th Army, concentrated on the Roslavl station. The unit remained here until May 25, 1944, after which it became part of the 33rd Army of the 2nd Belorussian Front. By this time, BePo-662 also received new materiel of the BP-43 type, though the replacement locomotive type is unclear.
This would imply the wagon you have pictured was lost in 1943/44, as the train was replaced then, and the Romanians/Germans took it over and repaired it. The commentary in the above link relating to Barysaw implies to me it was captured early 1943 and makes it from BePo-51 but this doesn't tally at all. It wasn't until the of 1943 that Pz IIIs were assigned to German rail wagon duties AFAIK, and the fact you can see it's a 4 axle wagon lends it to being the BePo-52 wagon.
Incidentally, the 2nd pic on the link shows a pristine train with 2x T26 armed wagons with no camo paint, so perhaps this was when first delivered, and then the wagons were split with the 2 artillery wagons from the other construction depot?
Incidentally UM-MT / Unimodels make a 1/72 model of this style, though with a few errors, as shown in this box art pic: