I'd like a train to Ely that leaves Wednesday.
Easy enough. Where is your departure point?
Cambridge. I would like to leave after 15:30 if possible.
TR6745 departs cambridge at 15:50 Wednesday, arriving at Ely at 16:07. Will that one work for you? It's only 4.40 GBP to ride.
Great can I book that for 8 people and get a reference number please?
You are booked for 8 seats on TR6745 leaving Cambridge Wednesday at 17:50. Reference # is OCWICAXZ and you can pay at the station.  4.40 pounds each.  
Thank you. That's is all I need. Goodbye.
Welcome. Have a lovely day.