Wikitravel says that you should bring warm clothing no matter what time of year, because of the differences in altitude:
http://wikitravel.org/en/Bolivia#UnderstandHere's a little graph of the average temperatures in Uyuni from Lonely Planet too:
http://www.lonelyplanet.com/worldguide/bol...yuni/when-to-goI think that will help.
As for the SLR, unless you are a professional photographer, or plan on framing your photos, I'd advise you to leave it at home. You'll take more photos with a pocket-size camera and technology these days means your photos turn out pretty great anyway.
Here's the website for the trains in Bolivia (I think) ...looks like the site's incomplete though...
http://www.fca.com.bo/I can't read it, but if you can, it should help you with the train situation.