Left alone, I'm a word with five letters.
I'm honest and fair, I'll admit.
Rearranged, I'm of no use to trains.
Again, and I'm an overt place, warm and well lit.
What am I?
The answer is "LIAR". After rearranging the letters, you can get RAIL - important for trains, or LIAR - a dark, hidden place. Since the riddler is a liar, the resulting words are exactly the opposite of his descriptions.