Deisel freight trains are huge poluters with regard to particulate matter (PM). This actually causes cancer as opposed to CO2. Reducing PM without impacting fuel efficiency significantly is possible but requires investment. https://www.epa.nsw.gov.au/-/media/epa/corporate-site/resour...
Same goes for diesel trucks / busses. This type of change throughout the world would really improve people’s lives immediately.
However PM is a fairly localized pollution that doesn't spread far. I don't think diesel trains that drive through the middle of nowhere are that bad. Near population centers it would be preferable to switch to electric locomotives.
There's a handful of hybrid trains in Japan that are electric in urban areas and use a diesel generator in rural parts of the route.
A quick search didn't reveal any running North America but considering diesel locomotives have mostly used diesel generators and electric motors since the 70s, it's not a stretch to think about implementation there.
Same goes for diesel trucks / busses. This type of change throughout the world would really improve people’s lives immediately.