Is there a train I can take to cambridge that leaves after 18:00?
Absolutely. Where are you coming from?
I'm departing from birmingham new street.
I have train TR8903 that leaves at 18:40 and will arrive in Cambridge at 21:23. Would that interest you? 
Yes please book me for one ticket.
Booking was successful, the total fee is 75.09 GBP payable at the station .
Reference number is : KHBLJGHN.  Is there anything else I can do to assit you today? 
Also looking for a guesthouse in the cheap price range.
Are you going to stay in a specific area of town?