Here is a nice explanation and rebuttal to an LA Times article on this from the CAHSRBLOG:
The argument Vartabedian, Ibbs, and other critics are making is that shorter stations make it harder to operate a “double” train set, as systems like Japan’s Shinkansen often do. But you could just as easily operate two single-sets with shorter headways. That would maintain your capacity.
Here’s an example. Sometimes the Shinkansen operates a single trainset:
And sometimes it operates a double set:
(Both gifs come from this video.)
Many European systems commonly operate single sets, such as Eurostar, the TGV, the AVE, and so on. So the CHSRA isn’t doing anything significant here in terms of capacity. But the savings is significant and welcome.
The same is true of tunneling. Lowering the tunnel speed from 220 to 200 mph provides a big cost savings on the cost of tunneling, but at a minor time penalty – that will likely be made up by other time savings elsewhere, including going almost directly from Palmdale to Burbank under the mountains rather than going via Santa Clarita.
So the CHSRA’s decision is sensible, as is often the case. But HSR critics will find something to criticize no matter what they do.
I would hope that even if platform length is limited to one trainset now, that provisions are kept to allow doubled trainsets later. I wouldn't imagine much would be required, just making sure there's enough space off each end of the platform kept clear to allow future extension.