Rhyl is a major holiday resort
Not since about 1964...
But it's certainly true that population size does not necessarily reflect usage.
Big swathes of the northern towns and cities turned their back on the railway in the 60s and have not come back.
Meanwhile seemingly unpromising areas like the Cotswolds, Devon and the North Wales coast kept their traffic up and deliver more passengers than their modest population suggests.
They are simply "more rail-minded", and travel more by train.
Also, have a look round and listen on the train.
There are far more students and non-English speakers travelling than there were.
These groups seem to use the train more than the resident population.
Demographics is a complex business.
Wrexham pop 42,000, usage 2012/13 - 657K, essentially flat (combining both stations)
Rhyl pop 25,000, usage 2012/13 - 612K, down 3.5%
Chester pop 120,000, usage 2012/13 - 3011K, up 1.8% (plus 805K interchanges)