Head-canon: There is some divine force hovering over Sodor, dishing out karmic comeuppances to anyone who gets a little too puffed-up.
Related: the Island of Sodor (and the larger universe of the RWS) is held under an interdimensional phenomenon such that its net change on the world around it is zero. Since The Railway Series is supposed to be exactly like real life except machines can talk and the Island of Sodor is an extra landmass that exists, there cannot be any differences between its universe’s course of history and real history. Its path of fate is locked with ours. It cannot deviate from the course set by its sister dimension. So yes, somehow there’s a whole new island that popped out of the ocean and the vehicles can talk, yet nothing else changes. No butterfly effect here. There’s an anti-butterfly effect anomaly in place.
they all know if they get too puffed-up, the karmic comeuppances will be catacylsmic
Sodor and Gomomah