Rainbow Dash is the only pony to every pull of the Sonic Rainboom, she has a loving colt-friend and needs to reflect in her usual spot. When her colt-friend follows her they both witness a horrifying crash of a pony going at full speed. Before that though they witnessed something that Dash found heart-breaking.
Ignore the chapter order, that's a screw up from me!
Half of the credit goes to Dark Lightning (Oops, spoilers, that might give you a clue to something in Part 1!)
Hit his page up right here- http://www.fimfiction.net/user/Dark%20Lightning
Thanks guys!