[RayChuang88]

If I remember correctly, one of the most famous of these "closed" sites was the factory and test stands for Soviet-era rocket engines.

Still in use today, what the Soviets extremely ingeniously did was hide all the production facilities and even rocket test stands so it looked like apartment blocks! The latter was extremely ingenious because they hid all the flame and noise damping systems underground so the test stands could operate without people noticing nearby. That was why the American intelligence agencies scratched their heads for years and years trying to find Soviet rocket engine test stands out in the "boonies" of the Soviet Union; in reality it was done at a Moscow suburb!

[GSlob]

Khrunichev site is too small to include large engine test facilities. I used to live in that vicinity. It is most famous for its employees [in the 70s] siphoning off a whole railroad tank car - 60 tons - of technical alcohol: they lifted a lid just a bit, so that the seals were not torn, inserted a long thin flexible tube, tossed the tube over the fence and hid the dispensing end in a big bush by the fence. By the time it was found, the tank car was empty. That's soviet ingenuity for you!