"I'm Mighty!" "And I'm Mac!" "And together we're Mighty Mac!"
— Mighty Mac's introduction, Mighty Mac, ninth season
Mighty Mac is a double-ended Fairlie duplex locomotive. The pair are conjoined male twins. 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.
Prior to their adoption into the Skarloey Railway family, Mighty Mac had only worked in the shunting yards. Their lack of knowledge beyond their boundaries created much conflict between the two halves (for example, which end is in charge of doing what) when they pulled their first train, which placed themselves and their passengers in great danger whilst running an important errand. But, with advice from Thomas in mind, they were able to complete their task and realise the value of working as one.
Mighty Mac is two engines in one. He is Mighty on one end and Mac on the other-- making him one of the most unusual engines working on the Skarloey Railway. When Mighty and Mac work together, Mighty Mac is the strongest engine on the Railway. But when they don’t, Mighty and Mac pull each other backwards and forwards, only to end up right where they began.
Mighty Mac is based on a Double Fairlie locomotive of the Ffestiniog Railway with added buffer beams. A Double Fairlie locomotive from the same railway appeared in the 1979 Annual. 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.