"Resiliency" is a good buzzword, but as with climate change adaptation nobody wants to spend money on it. It doesn't (usually) improve capacity for regular operations, so a private railroad (which pays taxes based on the value of its property and operations, rather than consuming them) needs a lot of convincing and an ongoing payment stream to consider it. Ordinarily, "resiliency" comes from having more than one railroad in a major corridor, and having commercial arrangements between them that allow traffic to be rerouted when necessary.

Using the Blue Cut fire for an example: both railroads and the freeway through Cajon Pass were closed for 24-48 hours. That's not really long enough to cause long-term problems for goods movement. BNSF mostly just parked things waiting for the tracks to clear; that's because, in part, they didn't have alternative routes to the LA Basin. UP had a couple of alternatives, so the fact that they had bridge damage on one line could be worked around (they're still planning to have the bridge fixed by next week per The News, which suggests as noted before by others that the damage was mostly deck and track rather than structural).

In all, UP has the bigger apparent repair job, though it's not too bad in the big picture; BNSF seems to have gotten off somewhat lightly. The freeway needed some repairs, but was open again northbound (outbound trucks from LA) in about a day. In all cases, the repairs look to be relatively minor. Cajon's a hot spot because as the only real low spot in the mountains everything has to go there, and both natural and man-made (most California wildfires are, accidentally or otherwise - no dry lightning storms there at the time it started) disasters affecting multiple facilities can easily occur. Resiliency for UP occurred in this case because it had alternative routes that were not affected (which Amtrak took advantage of even if a shuffle was needed at Mojave). BNSF and the freeway turned out not to be severely affected, but luck should not be a planning tool.

One other thing to remember: railroad tracks don't take long to build. So if, say, #BigOne was to the south and decisively knocked out both Cajon and the line to Yuma but left Soledad Canyon usable, I strongly suspect that a Mojave connection between UP north and BNSF east could be installed in days to a week at most to pick up the slack. And you could bet on a large response by the railroads to fix the lines that are broken by such an earthquake, though depending on structures and earthmoving getting them open could take a few weeks.

So ... resiliency ... is not only having connections ready, but being able to fix stuff quickly. U.S. railroads have a history of being able to fix stuff quickly, and in emergencies are usually ready to cooperate with each other. I'd be more concerned about the lack of such ability and planning on the part of the urban areas the lines have to go through to get to the ports and yards, and the effect on the railroads of long-term economic damage causing significantly reduced traffic.