When DMU (diesel motor unit) trains run through railway track we dont find any weights hanging from poles across railway track. But when EMU (electric motor unit) trains run through railway track we observe such weights hanging from poles.
ATD (Auto Tensioning devices)
This weights or device is called Auto Tensioning device (ATD), it is part of railway overhead electrification system. We know that during hot summer the metalic wire increases its length and during winter it decreases its length. The sag in the overhead electric wire can cause cut off electric transmission from wire to high speed train resulting entire route block.
To avoid such incidents or accident a 2 Ton tension / weight is maintained. Actual weight of these weight is 667kg but with the help of pulley applied load become 2 ton, 1 ton each wire. Elongation due to temperature variations is nutralised by this weight device.