On Saturday I need a train that will arrive by 21:45.
Can you tell me where you will be departing from and heading to?
I will be leaving from Ely and going to london.
Would that be London Liverpool or London Kings Cross?
Either one is fine with me.
Are you sure you don't have a preference?  
Yes I'm sure. I need to book that for 5 people.
We don't have any trains going from Ely to London Liverpool or London Kings Cross. Do you need anything else?