Idea momentum.

Sometimes they just carry the energy they have been generating forward like the groovy good-times brain train. The only problem I have with that particular analogy is the implication that those ideas are somehow bound to a track. It might be a fine thing to do, actually, since having a track further implies a known and connected destination. Anyway, the momentum is exciting no matter how you slice it... or roll it... while burning coal inside the engine of it.