A shunting puzzle from the model railway world. Draw a card, then make up that train in siding 1 — right wagons, right order — with one loco, three short sidings and a headshunt that never holds quite enough.
Inglenook Sidings is a classic of British railway modelling, devised by Alan Wright: a tiny shelf layout that is still fun to operate for hours, because the rules make every card a little logistics problem. Real yard crews solve the same thing every night, with longer trains and less patience.
Only the wagons on the card matter; the other three are interchangeable. That leaves about 356,000 possible arrangements of the Classic yard. When the page loads it runs a breadth-first search backwards from every finished train, so it knows the exact number of moves left from any position you can reach. That number is the "shortest" score, and the hints simply pick a move that brings it one step closer. Every card is checked to be solvable before you see it.