You can unlock a secret called 'Play as Beta Shadow Man' and enable it in the secrets menu in Shadow Man Remastererd.
This secret reskins Shadow Man to his appearance in the game's beta.
The Engineer's Key is required to journey in to the Cageways via train. Once you've taken the train to the main entrance to the Cageways or just warped there using Luke's Teddy Bear, you'll notice a series of ventilation tubes running through the area above the coal pit below. Make your way to the vent to the far right, just in front of the cable car and face the open tunnel ahead. You need to jump forward towards this tunnel and hitting it as close as you can. You won't be able to make it in, but the "Secret Activated" message will appear nevertheless.