I have a North Pole Express Big Hauler that I have had for a few years.  I set it up this year and it ran fine for a few days.  When I tried to run a few days ago, the power light on the power supply/speed controller goes dim as soon as I put the engine on the track.  What would cause that?  How can I fix?  Thanks in advance for any help you can give.  I miss running the train!!!   

Karen