For a screenplay, it is probably more important to be clear than to have excellent, flowing prose. (I'm not a screenwriter.) For the more-general case of descriptive prose, however, one approach is to convert "they are" verbal clauses to adjectival clauses. Instead of:
Hundreds of people are standing and looking at the on-coming train. Their sweating faces [are?] encrusted with dust, and they are wearing old clothes and shoes.
Try something like:
Hundreds of people, clad in old clothing and their sweating faces encrusted with dust, are standing and looking at the on-coming train.
(I also converted "old clothes and shoes" to "old clothing" there.)
How you do this depends on which aspects of the description are most important. You don't want to pile on a bunch of adjectival clauses (covering both the clothing and their faces in one sentence seems borderline to me), but as you've noted, you also don't want to string along a bunch of "oh yeah, and..." descriptive sentences. So focus on the ones that are most important for telling your story, and let the rest emerge naturally later -- or not.
To apply that advice, consider instead a longer passage like:
Hundreds of people, sweating faces encrusted with dust, stand and watch the on-coming train. (Something happens.) (Somebody watches) the sea of people, all clad in well-worn dusty clothes, swarm toward the doors. (Etc).
The point here is to add the descriptive detail as it fits, rather than loading it all up at the beginning.