- "Ladies and Gentleman, I give you Rosencrantz and Guildenstern, the most argumentative double fairlie on the Skarloey Railway."
- ―The Thin Controller, All the World's a Stage
The Thin Controller brought Mighty Mac to the railway to help out with passenger excursions, but the engines only had prior experience with shunting work. Mighty and Mac worked very indecisively at first, bickering and making it difficult for the other to get any work done. Eventually, they got over their feud and brought their passengers to their destination safely.
In "Two Hearts Burn Together", Mighty Mac got into an argument while they were working on High Tower Construction Detail, and the feud began to interfere with their other jobs. Eventually, when Mighty's swiveling engine broke down on a daunting incline, the Thin Controller told the engines their hearts and minds must be one to accomplish anything. Realizing their feud was silly, Mac did his best to carry on while Mighty offered some encouraging words. The two halves are now very good friends.
While pulling a passenger run by the lake, Mighty Mac was fired upon by Dex and Winslow. Mighty's cab was riddled with holes, and Mac's piston was in horrible condition. The two engines were rescued by Duncan, but were furious that it took seven direct hits for the hunters to realize what they were shooting at!
Mighty Mac perpetually argue, and during the Railway's production of Hamlet, they frequently bickered about which half deserved to faced the audience. To counter this, they were cast as Rosencrantz and Guildenstern.
When they met a stunned Arthur near a Man in the Hills carving, the two collectively explain the legend behind the etching while also noting they have been appearing more frequently beyond the original location at Green Hills Junction.
This is complicated because Mighty Mac has two different personalities exisitng simultaneously on one engine. The two halves are known to bicker constantly, but they almost always come to a resolution, at least until another argument arises!
Mighty is the older of the pair, identified by his swoop of hair. He acts like an older brother to Mac and believes he is far more cunning and handsome.
Mac is the younger of the pair who identified by his freckles. He likes to rebel and does not like to be told what to do by his other half. He is much more intelligent than Mighty gives him credit for, often saying something witty or devising a clever plan to get them out of trouble!
- Season 1: The Old Warrior (cameo), Double Whammy (cameo)
- Season 2: Two Hearts Burn Together, Pigeon Hunting, Scot-free (cameo), Mavis and the Tornado (cameo), Rosie (cameo), Just Another Wild Goose Chase, Rock-Star (cameo), Blunderbuss (cameo), Swan Dive (cameo), Luke and Millie's Blue Mountain Adventure (cameo), All the World's a Stage, Swashbuckler (does not speak)
- Season 3: Samson, BoCo, and the Man in the Hills, Chivalry is Dead