Mighty Mac is a double-ended Fairlie duplex locomotive. Mighty is the "older" of the two and is distinguished by a small flick of hair. Mac is the "younger" half and is distinguished by his youthful features, round nose, and freckled cheeks.
Despite the opposing personalities of the two ends, they are technically one engine.
Mighty Mac is based on a Double Fairlie locomotive of the Ffestiniog Railway with added bufferbeams.
Mighty Mac is 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're lined with red.