Think of Sacramento, Roseville, and Binney Jct (Marysville) as a big triangle with the Sacramento-Roseville leg at the bottom and Marysville at the top point. The Sacramento-Roseville leg is broken by the bridge fire, so all traffic that would normally use that leg has to go elsewhere. If it can't be rerouted around the area entirely, it has to detour up the Sac-Marysville leg then back down the Marysville-Roseville leg to get back on route.

Amtrak CZ is doing the detour; just adds an hour or 2 to what's already a late train so it doesn't really matter. Starlate often used the WP anyway (no stop in Roseville) so that's no big deal. Cap Corridor just ends all the trains at Sacramento (they were ending all but one there anyway) and uses buses beyond; probably no effect on schedules at all. San Joaquins are unaffected because the Y at Elvas where it turns off is between Sacramento and the burnt bridge.

Main freight problems are with runs that would normally end in Roseville for classification, equipment, or crew reasons. Many of the through Bay Area intermodals were going via Sac-Marysville (the old WP) to the Canyon anyway. I haven't seen traffic advisories from UP (their web site isn't one of the simpler ones for a noob to find his way around), but suspect that some traffic from the Valley and the Bay Area is now going south over Tehachapi and somehow ending up either on the Sunset or on UP's route to Salt Lake via Vegas. Some LA-Portland traffic that otherwise might have gone through Sac is almost certainly going Cajon-Daggett-Vegas-SLC-into Oregon. Other LA-Portland traffic, if it doesn't need work in Roseville, can just hit the WP in Stockton or Marysville and move on through. Doesn't really matter - if UP did do an advisory it would be something non-specific like BNSFs that are sometimes posted in the newsline here: traffic between x and y will be delayed by up to (n*24) hours due to congestion related to the bridge fire and reconstruction.

Bottom line - it's a major glitch that needs to be (and is being - remember my prediction of reopening in 2 weeks? - April 1 per the news for at least 1 track) fixed quickly, but there are ways around on existing lines for all but a modest portion of the traffic. No need for any massive projects to fix long-broken bridges or re-lay abandoned main lines for many more than the few 10's of million$ this repair will cost. Just a need for some experienced dispatchers at peak times and fingers crossed that not too many trains have troubles to foul up the works.