Answer:
Digital white-out.
If you drop me, I'm sure to crack. Give me a smile and I'll always smile back. What am I?
We are little airy creatures all of different voice and features one of us in glass is set. One of us you'll find in jet. Another you may see in tin. And the fourth a box within. If the fifth you should pursue it can never fly from you.
I am gentle enough to soothe your skin light enough to fly in the sky and strong enough to crack rocks.
I come with a train and go with a train and the train doesn't need me but can't go without me.
Dark, feathery, and popular in Baltimore and fantasy books.