It sounds like the worst option - 2 sites miles apart to see it all.
I was there last year and from memory the old trains were on the first floor (up). It must have been a job getting them up there and a headache if they want to take them away.
http://www.goldcoastbulletin.com.au/news/breaking-news/parts-of-powerhouse-museum-may-stay-in-cbd/news-story/2b395a2cba6d3cbc4e2e215d8ce5485c
Regards.