I think the original models used were G scale, with 0 gauge for the narrow gauge engines.
Awdry's writings were a great introduction to the world of railways and he always strived to ensure technical accuracy to his stories. From a modelling POV, reading the depth of history he wrote for Sodor is appladable and appeals for the modeller wanting to do something a little different. The MSR a mashup of Corris, Ffess and WHR practices, what's not to love?