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.