I'd like a train to London Kings Cross to arrive by 21:15.
Sure, which day would you like to travel?
I am travelling on Thursday and going to Cambridge.
Might I suggest train TR6595 leaving Cambridge at 19:00 and arriving at London Kings Cross at 19:51. Would that work for you? 
Yes, that does work.  Could you book it for 1 person please?
Okay, I went ahead and reserved your ticket for Train ID TR6595. The cost will be 23.6 GBP payable at the station, and your reference number is GKTT3OTZ.
That's everything I needed. Thank you!
You're welcome. Have a great day!