As Mike P noted, NWP as a Sonoma-Marin-Napa short line is now SMART and is a STB-regulated railroad aka common carrier. If another railroad connecting with it has no choice but to route freight traffic destined for it over SMART, SMART has to carry it. They don't have to make it easy or timely, but they have to move it if they physically can (lack of 286K capable track for loads requiring that might be a reason for requiring reload someplace, but if the traffic is reasonably presented and paid for they have to move it). And I don't think they can refuse the payment either; that's what being a common carrier means. Though UP and friends have definitely established that the rate quoted can be sufficiently unreasonable as to force use of trucks by any reasonable shipper.

The stories about the coal scheme seem to focus mostly on the Willits-Eureka segment aka Great Green Go To Pot Trail. There's also Cloverdale-Willits. SMART ends at the Sonoma/Mendo county line, and for practical purposes in Cloverdale. Presumably the Mendocino County line will be rehabbed too, but by whom? Both northern segments need to be dealt with before SMART has to really face the prospect of working around coal trains with their commuters.