Install the Pushover app, log in at pushover.net — your 30-character user key is at the top of the dashboard. It only lets this service send you messages; it is not a password.
Tap the button (easiest on the phone), drag the pin, or type the coordinates. Transit corridors are only a few km wide — pick where you would actually stand, not your city centre.
You'll get one Pushover message to confirm (nothing happens until you tap it), and every alert carries a one-click unsubscribe link.