Why could German trains not easily use Soviet railroads?

During Operation Barbarossa, German forces could not easily use Soviet railroads because the Soviet Union utilized a wider track gauge than the standard gauge used by German rolling stock. To accommodate their own trains, the Wehrmacht had to convert the captured Soviet lines from a wide gauge to a narrow (standard) gauge.1

Furthermore, as Soviet armies retreated, they employed a scorched-earth policy, which included the systematic destruction of railroad infrastructure and rolling stock to deny their use to the advancing German forces.2