This photograph is from the high speed line to the Channel Tunnel just north of the River Thames in Essex. You can see similar on the viaducts in Carlisle. They are to put tension into the cables to keep them taut. They are made of concrete and you put on different numbers, just like the slotted masses we use in the lab. Notice the intricate system of insulators to stop electrical conduction. And in this case you can call them weights, not masses, because gravity is pulling them down. I came across a fantastic quotation from Sir Patrick Moore: "Gravity is the force that gives weight to mass."