An idealistic young couple travelling by train on their honeymoon encounter a cynical elderly couple who turn out to be them in later life.

Genial!!! Perfecta historia y enganche!!!!

When do the lead characters know that the elderly couple is themselves in the future? (Is it by chance a big reveal?)

And then what?? It's an interesting situation, but the logline needs to follow through with a complete plot.? Try running it through the logline generator.