Multi-use cards allow you to build train cars, upgrade your engine, construct buildings, and pay for it all with the cards in your hand!
Each of the cards in Isle of Trains: All Aboard shows a cost to be built, a benefit when being built (like an ongoing ability or end of game scoring condition), and each card can be used as a good loaded onto a train.
There's no such thing as a bad card becasue they all have their uses!