It's not even all that difficult to estimate, really. For example, every business does it, of necessity.
Markets are a useful tool that societies use to optimize certain kinds of capital allocation and goods production, where that makes sense. No more no less.
Most of the things societies value most highly don't fit into the market hole.
If trains were exclusively a mechanism for capturing or avoiding carbon, then the metric of $ / tons would be valid. Trains very obviously are not that.
Where is this true?
https://en.wikipedia.org/wiki/Value_of_life
Numbers vary around the world, but FEMA in the US decided on $7.5M in 2020.
Financial investments are economic decisions. Whether or not you like the idea of assigning financial value to fuzzy concepts like happiness or quality adjusted life years, you still have to do it. The simple act of choosing to spend $N or not choosing to spend $N puts an implicit price on the result.
By putting approximate numbers in the spreadsheet - no matter how crude - we can at least end up consistent and fair. Otherwise we could end up spending vast sums to make a few people a little happier instead of smaller sums to make lots of people a lot happier.
This isn't even true. I run a business, and I frequently make decisions based on what I believe to be the right thing to do rather than what the data shows as being the most profitable outcome. The very notion of a "loss leader" is driven on fuzzy data where execs simply hope the loss in revenue from sales is made up for in other benefits that can't be concretely valued.
What is being suggested is comparing a train subsidy to... Unknown. Other modes of transportation? The money being spent on other carbon reduction efforts?
I don't understand why carbon reduction is the primary topic for discussion when trying to put a dollar value on the thing. Two apples are still a little difficult to compare but you at least can agree that the bigger tastier one is probably better value (unless the smaller is cheaper and you need to do gram for dollar value, idk, good luck). A train vs other things though is imo essentially impossible to compare, a genuine apples to oranges comparison.
For carbon reduction, I challenge the OP estimate. It assumes everyone drives if no train, as I understand it. So that's the carbon reduction comparison - wow great so many people not driving. But it's not just that. Less parking lots need to be constructed. Less concrete, less carbon. Less roads need re paving every 10 years or whatever - let's compare the carbon of rail maintenance to an asphalt road. Less car accidents, less cars needing to be recycled at plants. Maybe city designs start accommodating the train subsidy, less car centric design, more vertical and dense, less car travel that wouldn't have been served by the train but is now served by walking. On and on and on.
You can't compare these two on carbon alone, and that's not even considering the fact that I find it kinda ridiculous to focus on just carbon as a measure of worth-to-humans. There's so many other factors at play that I genuinely think it's impossible to put a dollar value on. Reduction in road noise for people near the highway. Reduction in smog and thus a reduction in lung cancer and related medical costs. Reduction in human deaths from car accidents. Increase in psychological happiness in commuters not exposed to daily road rage and also suddenly having more time to read or play games. Endless, endless comparisons.
Dollars are a bad way to measure value. We either need a new way to describe value-for-dollars or a new way to describe actual value. Conflating the two was capitalism's ultimate coup and this thread is a great example of why.
Yes, these are all valid other axes to consider. I nevertheless think it's necessary to estimate each of their values as so many "points", which, yes, may be subjective. Those points might as well be dollars, because dollars are what we (as the local government) finally wind up spending on whatever projects we decide on.
A teacher is more valuable than an investment banker and yet the investment banker is paid more. Maybe I can make a spreadsheet of all the instances of such things and it would reach tens of thousands of rows. It seems to me plain as day so I don't get it.
Sometimes the dollar value is accurate, often not. The frequency that it's incorrect makes me wonder at the authority we grant dollar valuations.
Because that is the comparison being drawn here. A valid objection would be that there are other axes that are material but are not being considered when we reduce the question to dollars per ton of CO2 -- but arguing that we ought not even try to put a value on how we spend public money will not be a reasonable stance until public money is infinite.