You're waiting for a train. A train that will take you far away. You know where you hope this train will take you, but you don't know for sure. But it doesn't matter, because we'll be together.
What's the implicit meaning of this riddle?
I don't see this explicitly as a riddle per se, but as a saying they came up with when Mal & Cobb knew they had to get out of their 50yr long dream.
The reason I say this is because they say it to each other to reassure themselves that by killing themselves in the dream, they'll wake up in the real world; portrayed as they say it while putting their heads on the tracks.
It is a sad way that they profess their love for each other because, while being stuck in the dream world for 50 years, you lose a sense for what is actually real (hence the totems) and obviously death is a scary thing for anyone to face, even if you're somewhat sure the outcome is to just wake up from a dream. They say this to each other to reassure that even if they're wrong and not actually dreaming, they'll be together in the afterlife, if not the real world.
It's not a riddle at all.
Cobb starts saying those things to suggest the trains exists to Mal. Notice that only when he starts talking to her about it, the tracks being to tremble and shake.
He is essentially causing the train to exist and Mal is conjuring it up.
There's a pretty well-known theory that the whole movie of Inception was Cobb's dream, as well as many theories about Ariadne being Cobbs therapist, etc.. but I choose to believe we, the audience were meant to believe the whole movie is Cobbs dream (I would recommend googling about this and deciding for yourself).
Anyways the quote is basically a fantastic "symbol" of 2 lovers in the midst of a midlife crisis. As long as they are together, it doesn't matter what "reality" they are in. Even after death, death is just a train taking them away together. You see this in how cobb can't get rid of Mal the whole movie. It doesn't matter where or what, Cobb and Mal will always be together. The "you know where you hope it takes you" is sort of that desire for being in a "true reality" that everyone feels. Then, the "but you don't know for sure" describes the contradiction in that we have no way of knowing what "reality" is as compared to a dream (if it were all Cobbs dream, totems wouldn't work or be a thing at all).
It makes sense once you know what the implicit question to the end of the riddle:
Why are you waiting for a train?
And the answer to the riddle is, because "you're waiting to die."
Cobb is incomplete. He's part in the dream world, part in reality. Split. The train is a metaphor for Enlightenment. To wake up from a dream but not knowing what will happen. A return to reality without description. Notice he asks Mal "Now! Tell me why?" at the end. In the now moment, it's said in true reality's now moment and there is no question. Then Mal says, "Then you'll be together!" Not "us". "Then you'll be together." One, complete, free.
It's a metaphor of life and destiny. The train represents the life path you chose, and although you may have certain expectations (or call it destiny if you like) you can never know for sure where that path will end. Yet, this uncertainty about life can be easily overcome when you have someone you love to follow that path along with you. After all, is not the end what matters, but how you live it and with whom you decide to share it with.
It was always my opinion that Cobb is trying to convince mall that dreaming is real and life isn't so they can fully immerse themselves in the fact that dreaming is real. That's why Cobb breaks into her mind and makes the totem stop. But then he can't convince her to go under again because she thinks she's ask ray dreaming. So she kills herself to escape the "dream" and tries to blackmail Cobb into believing it also/give him no other choice. The tragedy being that they both did it out of love but that's what ended up killing mal in the process.