Based on two derailment evacs that I have done,
3 days of clothing suitable for the season.
Know what hotels are on the other side of town or make agreements with friends on the other side of your town/city.
Have spare meds in your BoB along with some personal hygiene items.
If applicable, know what special needs items you will have to grab before you go. We keep a large suitcase packed with adult diapers, chucks, wet ones, etc. I know I have to grab the portable lift and both slings, and the potty chair.

There are a couple of motel chains that offer two room suites with kitchenettes for extremely reasonable rates and that helps us accomodate DD1 much easier.

Expect no warning of a derailment. My first experience occurred at 0400 and resulted in large propane cars exploding at about 1 per minute for hours. My second experience was at 1000 and involved tank cars of sulphuric acid. We were evac'd for 1 day during the first incident and 2 days for the second incident.

The above plus our BoB's and cash/credit cards is what we required this second time around. When we evac, we try to make it a vacation.