The unseen, human-voiced driver sealed inside the Yellow Line's locked engine car — who emerges only for a derailment.
The Engineer is the mysterious driver of the Yellow Line train, sealed in the magically-locked lead engine car that Vernon has never once entered or seen inside. Per Vernon, the engineer needs a Yellow Line Engineer's Key to reach, never responds to the (broken) intercom horn, is 'not a dwarf' and sounds human ('voices like a spike in my brain'), and — unlike the looping conductor — doesn't reset, seeming to change between runs. Vernon has heard of an engineer leaving the engine car exactly once, under one extreme circumstance: a derailment.
This makes the engine car and its hidden driver a standing mystery — and the key a coveted objective — for Carl's party.
Never seen; only a human-sounding voice over the train's loudspeaker.
An unseen driver behind a sealed door — a lingering Iron Tangle mystery the party wants to crack.