I have been looking at passenger count growth (based on ticket sales) for counties around the South West - looking at an average for the stations in particular areas, using DfT figures for the year to March 07 and for the year to March 08. 

For a Wiltshire station averaged 5% for the year ...
For a Hampshire station averaged 6%
For a Cornish station averaged 7%
For a Devon station averaged 8%
For a Dorset station averaged 8%
For a Somerset station averaged 9%
(in each case, a station geographically in the middle of the county was chosen, together with stations nearby)

You may feel that there's not much difference between 5% and 9% ... but year on year, it compounds.   Five years of growth at 5% gives an overall traffic increase of 28%, whereas a 9% growth over five years gives a 54% increase in traffic over a similar period - and those are (IMHO) significant extra traffic levels.   I expect FGW have done similar sums 

P.S. Cardiff = +9%, Slough + 9%, Swindon +5%, Evesham & Cotswold line + 2%... West Sussex + 7%, Surrey + 9%,