Telephoning to say where the train is (2,3,4)
I believe the answer is:
on the line
I'm afraid I can't explain this answer.
Can you help me to learn more?
(Other definitions for on the line that I've seen before include "Using the phone" , "At risk - or speaking on telephone" , "when one is at risk" , "Caller may be" , "At serious risk" .)