As I understand it, straight electric locomotives would use the 'dynamic' braking to send current back up the wires. Apparently this would make for entertaining economics-- a section of the rail network where most of the tonnage went downhill could produce a net negative power bill.

With diesel-electrics, there was nowhere to the braking power, so resistor grids were the order of the day. I wonder if it would be possible or worthwhile to outfit them with battery tenders to recapture the current with modern batteries and power-management circuitry.

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.