For some reason I couldn't get into the Railo Server admin on my dev box. The password was the same one I use for all my important accounts, password123!
To reset it, find railo-server.xml. On my Railo 4.2.1 Express it was here RAILO_INSTALL/lib/ext/railo-server/context/railo-server.xml.
The root node contains a pw attribute:
<railo-configuration pw="hashed-password-here" version="4.2"$gt;
Set that to pw="" and visit your admin page here http://domain/railo-context/admin/server.cfm where you'll be asked to enter a new one.
Or just remember it next time! Pass. Word. One. Two. Three.