The biggest change to "typical" GUMSHOE that Trail of Cthulhu has is the addition of a Sanity stat separate from Stability. Stability, like Health, shows how someone is doing right then; Sanity represents how close the character is to permanently losing it (in fact, this change separates Trail from all other X of Cthulhu games, since none uses two stats that way).
Cthulhu Mythos skill has an additional mechanic different from other Investigative Abilities, too. A player who is stuck can spend it and immediately find out a horrible truth driving the game - at potentially significant costs to Stability and Sanity.
I'd argue that Trail is a pretty stripped-down game, sacrificing mechanics for a larger list of skills. Since it is a pretty early game - as Steve says, the earliest one that hasn't seen a new edition - a lot of the changes in thinking about how to play GUMSHOE are sort of unofficial.