Depending on which way you look at it, Trade Route is either a useful card throughout the whole game, or a card that anti-synergizes with itself. The forced trashing ability is quite valuable early but a pain in the late game. But it doesn’t provide any money until the late game, and the +Buy is equally useless unless you have enough money to make it worthwhile.
In general, Silvers are preferable to Trade Route if there are other sources of card-trashing: Chapel and Steward are much faster, and Bishop and Remodel can provide bigger benefits early on. But deck-thinning is important enough that Trade Route is probably worth buying if there are no other ways to do so.
Naturally, Trade Route is most powerful when there are Victory cards in the Kingdom set, because those are usually popular enough that you can get at least some money off of the Trade Route early on. The presence of Hoard also greatly incentivizes players to pick up Duchies and Estates in order to gain Golds.
Trade Route is also useful in a hybrid Gardens strategy, taking the place of Woodcutter or Workshop. Even though it requires a card to be trashed, thus lowering your deck size, the +Buy helps make up for it. More importantly, a Gardens deck that concentrates on Gardens and Estates can guarantee at least +$2 on the Trade Route, and often $3 or $4 if Great Halls or Nobles are present. Opponents are also forced to buy Victory cards earlier than they would otherwise in Gardens games, further increasing the value of your Trade Route.
- Nobles, Gardens, Harems, Vineyards, and Great Halls
- Treasury, since people will be discouraged from buying Victory cards
- Chapel/Steward, since they are better deck-thinners, and Trade Route’s forced trashing is more likely to be a liability