The Train Wyrm appears, and indeed behaves, exactly like an 1800's steam locomotive. Travelling rails up and down the desert, the only way to tell it's not a typical train is that it has no conductor, no engineer, and no staff. It isn't until it's too late that it will reveal it's true nature.

Dragon

Blackened Steel

Steam Locomotive

Gigantic

It's cow grill is actually a deadly maw, it has one eye disguised as a headlight.

Steel, gears. and anger.

Passive until it or it's hoard are threatened, prefers to hide in plain sight. Watch out if you make it angry though.

Surprise attack, then fire and steam attacks.

Exterior: Hot metal and sharp points that jut out from it's cars.

Interior: A sort of "immune system" of creatures that attack intruders. Possibly kobolds.

126mph

Super quick and hard to catch. Can dish out massive amounts of damage in a short time period.

If it goes off the tracks, it's max speed slows to just 10mph. If it uses too much steam, it has to recharge and halves it's speed.

Choo choo noises,roars that sound like creaking metal, chuff chuff chuff chuff chuff chuff.

Gold, magic items and other treasures.

Located in the Desert/Western Ghost Town universe. Prefers long stretches of flat track.

Cows, other trains, unwitting passengers. Also water, coal.

When not trapped in CC, moves around a several thousand mile area at all times, stopping at train stations to be refueled by unknowing crewmen.

None. The origins of this creature are likely magic in nature, and so it is probably unique.