Mighty Mac are a double-ended Fairlie duplex locomotive, and conjoined twins from The Island Of Sodor.
Mighty Mac is two engines in one with Mighty being one end and Mac being the other. When the two work together, they prove to be the strongest engine on the Crotoonian Hills Narrow Gauge Railway. When they don't, the end up arguing and bickering and end up back where they started.
Mighty Mac is based on a Double Fairlie from the Ffestiniog Railway with added buffer beams. Three of these are currently in service on the Ffestiniog - another is preserved at the National Railway Museum, and another is under construction at Boston Lodge.
Mighty and Mac are painted royal blue with yellow and red lining. On Mighty's side, his cab windows and tank caps are lined with yellow, while on Mac's side, they are lined with red.