One of Newton's laws of physics is for every action there is an equal and opposite reaction. 

It therefore seems logical that if you could capture that reverse action when a train travels to a destination, then the energy for the return journey would be free.  Cut train emissions in half!

Is there a product that you would just love to own but no one has yet had the foresight to manufacture?  If so, why not let us know - you never know where it might lead.  OK you wont get the royalties if it turns out to be a storming hit but you will know that you had a part in helping to come up with a concept that reduced carbon emissions.