I have to travel soon and I have two options. One is to fly for 3 hours, another is to take a high speed train for 11.5 hours. The flight ticket is about 100 USD cheaper than the train ticket.
My aim is to minimize the environmental impact of my travel. So taking the train seems to be the obvious choice. But I wonder if I'd better save the 100 USD and donate the money to organizations such Clear Air Task Force or Sunrise Movement, which may actually have better impact on the environment?
True; this still means you're doing something with the "profit" from that extra time and not just letting the information sit in your head. You're putting it into an impactful job (and not playing videogames) or you're using the money to mitigate the damage.
I think there are at least two points against believing this.
First, you're directly harming the world in a specific way by flying instead of taking the train, and you don't want to take a moral position where it's ok to harm some people in order to help others "more effectively".
Second, some cause areas lots of people here believe in are enticing in that investing in them moves the money back to you or to people you know, instead of directly to those you're trying to help. Which is not necessarily a reason to drop them, but is in my opinion certainly a reason not to treat them as the single cause you want to put all your eggs into. It's easier just to see them as the most moral, no matter the circumstances, but I think that's dangerous.