*Lexi is a boisterous American cab-forward engine who works at the Steelworks on the Mainland.

When Thomas travelled to the Mainland, he befriended the experimental engines: Lexi, Theo, and Merlin. Thomas encountered Lexi and Theo in an empty yard, where he was warmly welcomed and refilled with coal. Later, Lexi, along with the other experimental engines, helped Thomas rescue James, who had been trapped in the Steelworks.

Afterwards, Thomas encouraged Lexi (along with Theo and Merlin) to stay with Frankie and Hurricane and call the Steelworks their new home.

Lexi is an eager and excitable engine. She is proud of being experimental and 'cab-forward'- even though she is convinced this makes her an un-useful engine.

Lexi is upbeat, cheery and outgoing, and likes to experiment with how she presents herself, often trying out different voices and how she phrases her expressions just to see which one she thinks works best. Lexi is not always confident she gets things right, but she loves the excitement of anything new, is very sociable, and keen to explore new things.

Lexi is based on the North Pacific Coast Railroad (NPCRR) Cab-Forward 4-4-0 No. 21 "Thomas-Stetson", a rebuild of NPCRR #5 which had been wrecked in 1897. An innovative engine built by Bill Thomas, it is considered the first cab-forward engine and was one of the first oil burners, as well as the first engine to use a marine boiler. The engine had insufficient adhesion weight and its oil burners were prone to damage due to their proximity to its water tubes, and it was unpopular with crews, and its unusual appearance earned it the nickname "The Freak". #21 was scrapped in 1905.

Lexi is painted light turquoise with a gold cowcatcher. Her front windows have gold lining, and her side windows have copper lining. Her main body is covered in rust. She has brass nameplates on the sides of her cab and her former company's initials "N.P.C.R.R" are painted on her rear oil tank in white.*