Some diesel electric trains now have large batteries and can recapture the braking power. Though this is seen as a bonus; the primary goal of the batteries is generally to be able to switch off the engines in station to reduce local diesel emissions.

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.

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.

IIRC there are mine trucks set up similarly in some locations, they're loaded at the top, regen downhill, and that's sufficient to power them back up the hill when empty.

On the other hand, pure electric trains seemingly have had regenerative braking for a hundred years.

A diesel electric locomotive has no serious batteries, and no room for enough batteries to consume the energy of slowing down a train.

At least it can dump it as heat without also producing fine dust, like mechanical brakes do.

They can (and do) have room if they're designed from the ground up for it. The engines for diesel electric trains are so large because they need to be sized to drive power for peak energy (usually accelerating and hills). If the energy can be stored, the engines for hybrid locomotives themselves can (and are) smaller.

So far you're only seeing hybrid locomotives for trains that stop/start a lot (shunting trains and passenger rail). The cutover for freight will likely take decades because A) locomotive lifetimes are measured in decades and B) longer range freight usually has less stop/start, making it's economical delta less.

I would imagine long range freight would be more likely to have long stretches of uphill grade which would mess with the minimum battery size

Yeah, that could be a problem. Even modern trains often now have multiple locomotives (including in the middle of train sets) to deal with range/weight, so who knows. There’s no reason hybrid trains can’t have multiple locomotives or battery bank-cars for those situations.

As I mentioned above, the freight lines are very conservative with new tech and amortize equipment over decades (often to the point where many rail cars are unsafe, especially outdated tank cars), so even if it exists we won’t see it in practice for awhile yet.