Comparing Renton to this situation is like comparing an anthill to a skyscraper; the Renton line was a low-traffic line (north of the Boeing plant, at least) that BNSF wasn't too interested in keeping, while the Sacramento route is a major mainline that UP will never give up unless there's a radical shift in its traffic patterns. Matt K is quite right for all of the reasons he's listed; UP certainly won't consider this idea, and neither would any other railroad or operating agency. (And if my remarks suggested that I support a hare-brained scheme like this, then I apologize; I was merely pointing out why these people brought up the idea in the first place. Yes, it would be a good idea to have direct pedestrian and bike access between those areas, but not by using a very active railroad ROW that the owner has no intention of "giving up" to anyone for any reason.)