My son got the Lionel Sante Fe starter set for Christmas. All worked well until today. I setup some MTH RealTrax just like the 4th layout on this page:
The RealTrax was purchased used, so I cleaned it with some track cleaner. I had previously made a small layout on the floor using the RealTrax and everything worked great.
Today, after getting all the track done, the train hardly runs, and when it does, the headlight flickers constantly. Stopping in many areas, when the train stop the headlight is very dim, but on. Sometimes tapping the surface (using my 4'x5' plywood base) will allow a good connection to happen and the train to operate. As a test I used the FastTrack that came with the train and setup the oval loop and I had the same things happen.
On a whim I put the train on a cradle upside down and applied power directly to the engine. I was getting the same headlight flickering, dim headlight, intermittent operation as I was on the track. So, I don't know if the problem is the track or the train itself.
I'm guessing a call to Lionel is in order and possibly sending the unit back for some factory testing.