Chasing rainbows

For about an hour the train seemed to straddle night and day. To the right the sun continued it's slow descent into dusk though a clear azure sky. To the left thick black clouds threatened to flood upon the countryside beneath, still glowing in the light of the falling sun. A rainbow arced gracefully through the skies, a thing of rare beauty shining brilliantly against the darkness. It seemed to follow the train, vanishing momentarily from sight as the carriage dipped behind a low hill, reappearing seconds later. And then, almost simultaneously, the clouds thinned and the sunlight faltered. The rainbow vanished, leaving naught but a memory behind it.