I would like to find a train leaving Cambridge after 6:30 PM. Can you help me?
I would be happy to help you today. What is your destination?
The destination is stansted airport, and I would like to leave on Sunday. 
I have a 06:40 that arrives Stansted at 17:08. Would you like me to book that for you.
I apologize. I may have confused you a bit. I would like to leave after 18:30. 
There is no train leaving at the time you require,unless you try a different day,would you like to try a different day?