An example of such a line is in Sweden https://en.wikipedia.org/wiki/Iron_Ore_Line

> From Riksgränsen on the national border to the Port of Narvik, the trains use only a fifth of the power they regenerate. The regenerated energy is sufficient to power the empty trains back up to the national border.

Similar thing happens in Australia: https://www.jalopnik.com/these-electric-trains-never-need-re...

The twist: these trains aren't connected to the grid. They use regenerative braking to charge batteries when carting ore to the coast, and the batteries power the trip back to the mine.