What is the implicit meaning of Mal's riddle in Inception?

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.