Siemens went all-in on US production, and seems to be making a go of it. Alstom & Bombardier also have semi-permanent operations in the US. Others pretty much set something up for particular orders requiring US content, then shut down when they were done; sometimes, if they got another order later, they set up in another location. Still, I doubt that even Siemens could make it work as a US-only operation - as part of a global conglomerate, though, the slow periods can be filled with work from elsewhere.

As for Amtrak orders, the Brightline trains don't fit any current Amtrak or state-operated-Amtrak specs. However, Siemens did repair work on some Cal Cars at one point (though the big rebuild contract went elsewhere) and could certainly come up with something similar. They also seem to be pretty big in the rail transit repair/rebuild business, which fills in some holes in the work schedule. And remember, in the US it's easy to lay off staff when work gets slow; much harder to do so in Germany.