It's a red flag with a steam train on it. How cool is that?

The picture is from a recent visit to a railway museum where they had an exhibit about the cold war. Being written by western libs the text next to the flag talked about how civil defence at places like the railway workshops was complicated when "everybody didn't agree on who the enemy was" and called the communists a "fifth column". Apparently, as all workers had to take part in civil defence still and know about plans in case of war, authorities were worried that the large communist presence at the workshop meant that they would tell the USSR about the plans or use their knowledge to sabotage the railways in case of war.