How do you get from harlow to northampton train station?

Depends where you are in Harlow. Easiest way is to get a taxi as they will come and pick you up and drop you off at the station. Assuming you are talking about England, Harlow, Essex is about 75 miles from Northampton - a very expensive taxi ride. The quickest way is to drive via the M11, M25 and M1 motorways. Going by train would involve going into London and out again. There may be a bus between them...