train won't reverse

I have a lionel norfolk and western set. It goes forward only using the transformer it came with even though the transformer has reverse capability. I have discovered that it will go in reverse using a dc direct power source. Why doesn’t it go in reverse with the transformer it came with.

sounds like a Lionel engine, they usually have a switch which enables reversing, otherwise they will never reverse.
cool that it runs on DC and can reverse.