A Coat to Keep Her Warm

[Magpie]

This is interesting. You've got a good idea that needs a little expanding but has potential.
I've left a few thoughts below.

(This is my first post in a number of years. Please tell if I'm not doing it right).

She was so old
she could call back the days
when rumbling trains would cross the land,
when she would sit in her seat   - don't think you need 'sit in her seat'
and see the brown the black beasts  - do you mean 'brown and black' or either or? 
run like waves of sound.  - this simile doesn't work for me. I get  the 'moving like waves' idea to express immense numbers
She saw them fall too,
silent lead from invisible rifles - not sure what 'silent lead' is? gunshots aren't silent. Would 'unseen rifles' work better perhaps 'invisible' seems intangible 
thumping into heaving hides,
final snorts of life rippling the golden grass. - two good lines as regards imagery - 
Then there were no more.
The plains died,
the beasts dried to white.
She kept her coat though.
It kept her warm when the fires waned. - these last two lines seem awkward. it's a good end, just needs slightly rewording - repetition of 'kept'

I like the poem. I like the idea. I am left wondering who 'she' was/is and perhaps this could be expanded upon as an angle?? 
I get that it's about mass killing of bison. Perhaps everyone will get it, but it does require some knowledge of American history to a certain degree... maybe a different title could provide a clue.
all just thoughts

thanks for the read