Hitchin: 2,580,000
Stevenage (used more because expresses stop there): 4,258,000
An interesting breakdown of figures. Stevenage isn't all that much bigger than Hitchin, but gets double the number of passengers because of the expresses.
Begs the question as to which stations have the highest population to usage ratio? I'll give you a minimum population figure of 1000 to stop some clever so-and-so bringing out Sellafield or some such example!
I'll start you off - Kendal Oxenholme ward has 2206 people; Oxenholme Lake District got 0.418 million passengers in 08/09: that's a ratio of 189.483228:1