There is a new place to get a great look at the trains while they are underway.
As part of the renovation of the Space Mountain entrance area, a section of the shrubbery has been replaced on the right side of the track just past the Space Mountain entrance/arcade building. There is an open area of concrete surrounded by a concrete 'bench' that is just behind the Tomorrowland restrooms (access between the restrooms and the Space Mountain entrance). It looks like it is intended to be a 'smoking area', but few people were using it for that when we visited last week.
However, for now at least, the only thing between the 'bench' and the tracks is a planter with new (and so far, very small) bushes. If you stand on the 'bench' you get an excellent, and very close, view of the trains pulling up the hill to Main Street.