Wall Street Journal today has article–
Wabtech is experimenting with battery powered electric locomotives. So far they are mostly designed to charge batteries from dynamic braking which currently sends power to resistors to discard as heat. That should result in better fuel efficiency. Battery electrics may be feasible in some yard operations or port service. Slow charging is a problem.
Biodiesel has potential to replace part of the diesel fuel but supplies are limited.
CPKC and CSX are testing hydrogen fueled locomotives. They are using fuel cells to generate electricity. This could make for green locomotives using green hydrogen. The storage problem is not discussed. Compressed hydrogen requires heavy steel cylinders limiting range. Liquid hydrogen is more concentrated but requires very low temperatures. Usually maintaining that temp requires allowing the hydrogen to boil which emits flammable/explosive vapors.
In the steam locomotive era, tenders were included to carry fuel. That could be done with hydrogen tanks or even with battery tenders. Battery tenders could be switched out at crew change intervals and replaced with charged tenders–pony express style.
The article is a bit unusual in that Caterpillar’s EMD division once part of GM and formerly the leader in locomotives is not mentioned. What are they up to? Wabtec (sometimes Westinghouse Air Brake Co) purchased the industry leading locomotive division from GE a few years ago.
EMD is now part of Progress Rail subsidiary of Caterpillar. They are less out front about it but are working with CN on a hybrid electric: https://www.progressrail.com/en/Company/News/PressReleases/CNContinuestoAdvanceDecarbonizationEffortswithEMDMainlineHybridLocomotivefromProgressRail.html
EMD is supplying battery electric locomotives for evaluation by BNSF in California.
https://www.progressrail.com/en/Company/News/PressReleases/ProgressRailtoSupplyBNSFwithZeroExhaustEmissionLocomotivestoDemonstrateinSouthernCalifornia.html
Wabtec is easily the best performing railroad stock at the moment.