Those are things that regulate the tension in the overhead cables on train tracks. With the variation of temperature cables tend to contract/expand and this systems allows the cables to do so freely with a constant tension provided by the weights.

In case people don't want to click a link, let me explain it here:

If you want to use overhead line electrification, you need to suspend a wire over the rails. In theory, you could simply hang up a wire, but whichever amount of tension you choose, if it's warmer outside, the wire will droop, potentially causing damage, while if it's colder outside, the wire will pull taut and may snap. So you want a system to account for external temperature.

Instead of picking a tension at a standard installation temperature, you pick an amount of desired tension and use weights to pull it taut. Now, if the wire heats up and extends, the weights drop, and if the wire cools down and contracts, the weights are pulled up.

And to keep the amount of weight you need to add under control, you use a series of pulleys to control the tension in the wire.

In NL, the mainline system looks a lot simpler: They have only one wheel, but that's two pulleys: a larger one and a smaller one. The larger one holds the weight, while the smaller one holds the wires.

The train duh