This online article says
Soviet Union had a different gauge rail network and the Germans operated much heavier trains than the Russians, so when the Germans invaded they had to rebuild or modify the lines everywhere they went. Some of them were converted to standard gauge by moving the rails. The Germans replaced the Russian wooden sleepers with steel sleepers...
It has a lot of photos and talks about the Rail Plough used to destroy railroads as they retreated. However, I question the use of steel sleepers. Steel was surely much more expensive and rare than wood.
And there must be 1,000 or more sleepers per km of track, and Germany penetrated almost 1,000 km into the Soviet Union during Barbarossa. Would Germany really be spending that much steel, when they could just use wood?