Taking a far more localised look at growth rates, I've taken the TransWilts journey numbers for the 2016 (9 months) and 2017 (3 months) period, and compounded them at various rates - firstly the 0.8% rate on which forward planning from 2006 to 2016 was based, then some rates that would reflect some of the achievements around the South West in the past decade
In reallity, growth rates will change from year to year, constrained by both supply and demand, and muted in how quickly it changes by the time it takes people to tie up something new (quite slow) or give up on something that doesn't work for them (quote fast, I fear).
On TransWilts, the current constraint is supply. By various measures, latent demand is far in excess of that supply; we're getting to be full and standing in the "shoulder" as well as in the peak, and so much feedback suggests more passengers if some of the gaps.
* the train prior to the 17:36 is 144 minutes earlier on a "local" service!
* gap from 06:48 to 09:33 in arrivals at the county town of Trowbridge which rules out commuter traffic on the line to there from Swindon, Chippenham and Melksham
* later train - last train from Wesrbury 18:32 on Saturday, and from Swindon 20:06 on Monday to Friday
Demand is such that (I believe) the 15% growth rate could be achieved on average over the next 10 years. It's hard to look that far ahead, but supply is such that I would be very surprised to see anything like that rate achieved without "Vision 2020" implemented.
