Twice yearly there was a Tri-Mission operation...to count the old ones out and, the new ones in. Up until the 80's they would move via the railways,in Mannschaftwagens (similar to the jobbies in Dr Zhivago). We would sit in OP's on specific rail lines (whilst trying to avoid Trapos),for 24 hours counting anything military that moved.As was said to me many years ago. The more pieces of the jigsaw you have, the better the picture looks.
After dark, you moved closer to the track to get a lower viewpoint against the night sky,giving you a silhouette of the wagons and,any kit. At a pre-arranged time,one tour would come off a line,whilst another tour took up an OP somewhere else along it. This normally went on for about 3 weeks.
In the 80's they started flying them in using Ilyushin ll-76 'Candid' into specific airfields,which made counting slightly more civilised.