Great idea! A couple of suggestions:
1. Make an option that is formatted for text messaging (under 160 characters, text-based without HTML, etc.) All the major carriers have an email address tied to the text messaging on your device (for example,
[email protected] will send that email to phone number 712-555-0123, assuming that person is an AT&T user), so that could be a way to get alerts to cell phones without having to deal with some sort of text messaging interface.
1a. Coupled with that, have a way to subscribe/unsubscribe via replying to the email.
2. I'm assuming this is true, but have a way to make the alerts station-based. For example, I would want to know when the train to OMA is late, but not that same train coming into EMY.