I suspect the thermal issue can be handled but not without some advance planning. There was a "what is it" item here several months ago that turned out to be an expansion joint for use in CWR - suspect those will be needed.

The real problem in the Valley is the RANGE of temps; winter mornings are frequently in the 20s and teens are not unheard of, while summers have long stretches of 100-110 (or more at least once or twice a year). Luckily, the DAILY temp range usually sits around 30 deg F (110 deg day drops to around 80 at dawn; 100 to low 70s; etc.) so as long as the weather doesn't swing too far too quickly the track can probably adapt. Those fancy joints probably are needed to handle the climatic change over the seasons, though.

I'm not sure whether restricted speeds on Sac light rail on hot days are due to track concerns or power problems (both are possible). Maintaining adequate wire tension seems to be difficult on very hot days (catenary has been pulled down by light rail trains in Sac on 110 degree days when it went too slack) - not an unsolvable problem but something else the engineers need to think about.

Bottom line - yes, things could get tricky for system design in the Valley, but nothing really new is needed to deal with it, probably. It gets pretty hot in the South of France, too, and central China can go through a really wide annual temp range, but HSR works OK in both places. I'd be a little more concerned that they engineer the active fault zone crossings (of which there will be several, of several different types of fault) for some safety margin and repairability. We may need some advice from Japan on handling earthquake monitoring and response for HSR operations.

FWIW, highways can have temperature problems too. Concrete freeway pavement has been known to buckle when the weather gets too hot too quickly. So this is an engineering issue not a project-killer.