Obsolete and lacking spare parts. One of the quirks of the Stalinist 5-year plans (Stalinist and others) was that the targets were in terms of absolute production figures of capital items - so many tanks, planes, tractos, trucks, etc. If a plant decided to build up a stock of spare parts, then they would not be producing items for the target figures.

Therefore, a number of tanks etc became immobilised because of even minor breakdowns for which simply there wouldn't be spare parts in stock.

This is one of the beauties of a planned economy, Communist style.

I remember a joke about this, that went something like: "Suppose a Soviet nail factory. If the target were in number of nails they would make only very small nails as you could get the most of these from each ton of iron. If the target were in production weight, they would make only very large nails. If the target was in production value they would make nails in gold".

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