Oh hey here's a Trail interaction that is actually specific to the card-shaped thing it's interacting with, and also confusing enough that it kind of needs a short guide.
Trail + Way of the Mole
The normal version:
Step 1: have Trail and any other action in your hand
Step 2: play the other action as Way of the Mole, discarding Trail alongside the rest of your hand
Step 3: you now have 0 cards in hand, use Trail's reaction to play it as Way of the Mole to draw 3 cards
Step 4: you now draw 3 cards from the original Way of the Mole, getting you to 6 cards (and one more action than you started with)
The version on steroids:
Step 1: have at least two Trails and any other action in your hand
Step 2: play the other action as Way of the Mole, discarding two Trails alongside the rest of your hand
Step 3: you now have 0 cards in hand, use Trail's reaction to play it as Way of the Mole to draw 3 cards, while the other Trail reaction is still unresolved
Step 4: you get lucky and one of the 3 cards you drew is another Trail
Step 5: you resolve the other Trail reaction to play it as Way of the Mole to discard your hand again and react with the Trail you were lucky enough to draw
Step 6: you now have 0 cards in hand, use Trail's reaction to play it as Way of the Mole to draw 3 cards
Step 7: you now draw 3 cards from the Way of the Mole from step 5
Step 8: you now draw 3 cards from the original Way of the Mole, getting you to 9 cards (and three more actions than you started with)
Since steps 4-6 of the steroid version are in fact equivalent to steps 1-3 of the normal version (with an extra unresolved +3 cards at the end), and you can extend the normal version into the steroids version by having an extra Trail in your hand and drawing an extra Trail with the topmost +3 cards after discarding, every two extra Trails you have in your deck can increase the maximum handsize you can get this way by 3, as long as your draws are good enough. Conveniently it also doesn't matter which specific hands the extra Trails are in, as long as they're in some of them before the chain ends, and the more Trails you have, the more likely you are to keep drawing more where it's required.
So basically with Trail and Way of the Mole in the kingdom, it is possible to build a deck-drawing engine without any other draw or +actions.