Ok, so here's the list of main Soviet tank producing centers before the war:

• Moscow - plant №37 (produced light amphibious tanks - T-37a, T-38, T-40, ~4.500 total)

• Leningrad - plant №174 (in 1931-1941 produced light tanks T-26, total production ~10.000, and started light tank T-50 production in 1941) and Kirovskiy plant (produced medium tanks T-28 and heavy tanks KV-1 and KV-2)

• Kharkov - plant № 183 - (produced all tanks of BT series ~8.000 total, all heavy T-35 tanks and almost all pre-war medium T-34 tanks), also have to be mentioned plant №75 (produced all diesel engines V-2 - tank engine for T-34 and KV series tanks).

Stalingrad Tractor Factory is not mentioned, because it failed to become real tank factory before the war (only ~180 T-26 produced in 1933-1940 and ~200 T-34 in 1940-1941)

As you see, Russia had far more than one tank factory before the war, but wait, it gets more intresting.

Soviets started evacuation of industry, when German troops came close to those cities.

So:

• Moscow plant №37 was evacuated to Sverdlovsk.

• Leningrad plant №174 was evacuated to Omsk, Kirovskiy plant was evacuated in Chelyabinsk

• Kharkov plant №183 was evacuated to Nizhniy Tagil, plant №75 - to Chelyabinsk.

Despite the evacuation said factories kept production and repairment of tanks with remaining facilities (for example some T-35 tanks were still repaired in Kharkov, when city was captured by Germans in October 1941).

Stalingrad Tractor Factory became important during late 1941-early 1942 (when evacuation of other factories haven't been complete yet) as main production center of T-34's. Leningrad was blocked and it was impossible to send off KV tanks anywhere but to defend Leningrad.

Also Soviets started production of T-60 tanks (mainly composed from automobile parts) on GAZ car factory in Gorkiy, plus another factory in Gorkiy - №112 started T-34 production in 1942. Uralmash factory in Sverdlovsk started T-34 production, also Moscow plant №40 was converted to SPG production.

So in 1942 there was tank production in:

• Chelyabinsk (Kirovskiy plant + plant №75) - KV tank series + SPG's.

• Nizhniy Tagil (plant №183) - T-34 tanks.

• Omsk (plant №174) - T-34 tanks.

• Sverdlovsk (Uralmash - T-34 tanks, plant №37 - SPG's).

• Stalingrad (Tractor Factory) - T-34 and T-60 tanks. Despite German advance, production stopped only in September when Germans broke through to the factory.

• Gorkiy (GAZ - T-60, T-70 tanks and SPG's, plant №112 - T-34 tanks).

• Moscow (plant №40) - SPG's.

I hope it answers the question about "only one tank factory", and i'm not even mentioning numerous factories, converted into tank repair plants all over the Russia.

About sending tanks from the factories immidiatly to the front line. I hope you understand that before shipment those tanks were properly tested first, each factory had its own shooting range, for example. But sending tanks to the battlefield implies that tank crews patiently waited for these tanks to be produced at the factory, and this is BS.

Probably it was true for Leningrad and Stalingrad - close proximity of the front line kinda suggests it (i'm not even talking about impossibility of sending tanks somewhere else in case of Leningrad and Stalingrad in August-September 1942). It would also be true for any tank-repair facility in close proximity of the front line (it's kinda logical to send tanks in need of repairs there, isn't it?).

No idea about rebuilding some factory brick to brick though, sorry.