Your statement is like saying "why should we spend money on new cars when people are dying from cancer."
Smart.
What are you talking about? Funding from state, local and federal transportation sources spent on this does take potential funding from infrastructure improvements. Your statement makes no sense. Not smart.
https://www.railstotrails.org/build-trails/trail-building-toolbox/funding/acquisition-funding/
There are both federal and state funds set aside for this, in addition to what can be raised from private or corporate sources.
What usually happens (at least in the ones I have watched in my extremely blue state) is that various criteria are met (sometimes it is a certain level of private/corporate donations or town funds) which can then be sent to both state and federal levels for additional funding from those sources.
From what I have seen, these are almost never built without the assistance of town, state, and even federal funds.
So that money IS coming from somewhere. I would even go so far as to say the federal funds may originate in federal highway funds allocated to a state, which then doles them out to towns to build things like rail trails.
In my specific case, I KNOW money that could have been spent repairing roads was spent on this Rail Trail. Again, I love the Rail Trail concept. Putting land to better use, giving safe and healthy avenues for exercise, enjoying nature, etc. I get all of that completely.
I just think that fixing roads we MUST travel on should take precedence, and Rail Trail Federal and State money should only be allocated when it does not increase our national debt...which it is indisputable that it does.