I believe the answer is

LIARS

The riddle was tricky (as indicated by the title) and each of the lines besides the first is

a lie.

I'm honest and fair I'll admit

Obviously, a liar is the antithesis of honest and fair.

Rearranged, I'm of no use to trains.

RAILS, an anagram of LIARS, are incredibly important to the function of trains.

Again, and I'm an overt place, warm and well-lit.

LAIRS, another anagram of LIARS, are hidden, dark places, the opposite of the adjectives provided.

I could be way off track but I have a good feeling that this is it.

I believe the answer is

LIAR

The riddle was tricky (as indicated by the title) and each of the lines is

a lie.

Left alone, I'm a word with five letters.

The word is actually 4 letters

I'm honest and fair I'll admit

Obviously, a liar is the antithesis of honest and fair.

Rearranged, I'm of no use to trains.

RAIL, an anagram of LIAR, is an incredibly important facet to the function of trains.

Again, and I'm an overt place, warm and well-lit.

LAIR, another anagram of LIAR, is a hidden, dark place, the opposite of the adjectives provided.

I could be way off track but I have a good feeling that this is it.