Little information
From The Lost Media Wiki
23) The crew also began making more use of the larger sized engines in Series 6 for the Pack episodes?
For the film made in Canada (Magic Railroad) they used the normal Gauge-1 as it would have cost too much to rebuild everything at a larger scale. They did build Thomas and Percy at twice up from the original scale to test with, but it was only for these two. They eventually got used with Jack and the Pack as this was made to a scale of 20mm to the foot. The Thomas series moved to Digital film media around 2002-2003 and it happened that we first used it on Jack and the Pack