Without a lot of extra green cards, and before many green cards have been bought, I think it's probably more correct to look at Trade Route as a trasher first and buyer/earner second. The potential gold-as-action later in the game is significant, but by then, unless you're in a cursing game, you'd probably rather have actual gold. So, earlier in the game, I'd think primarily about how much you want its trashing ability. Which, in the average kingdom, is probably too weak to go for on that basis alone.
I remembering reading here that Trade Route is "kind of" worth using when there's even one token on the trade route mat, assuming you have stuff you'd like to trash and don't have any other trashing ability. But I wouldn't buy an early Estate or Duchy just to put that first token on. If someone else happens to do it, though, it might be worth taking advantage of it. With two, it becomes a more compelling choice.
It might not be completely out of line to evaluate the desirability of Trade Route (in three-victory-pile games) similarly to how you'd evaluate the need for Steward. Steward is more powerful at trashing and (eventually) less powerful at earning, but both cards provide both benefits, which enables them to be useful both in the early game and in the late game. Probably Trade Route's median point of usefulness is a bit later than Steward's, but the overall spread on both cards is pretty broad, I think.