Squashed commit of the following:

commit 9cb8b3005d0a402b0927e4da7f37291a863b0ea8
Author: Nik Daftary <nikdaftary@gmail.com>
Date:   Fri Sep 20 11:20:48 2013 -0500

    comments

commit 72e184fc8ebf51845ebb037314d1e734f064fbb8
Author: Clare Glinka <glinka.cb@gmail.com>
Date:   Fri Sep 20 11:15:10 2013 -0500

    Add Train#trip_distance