Peter Sam, named after the Skarloey Railway's previous controller, Mr Peter Sam - originally known as Stuart, after his builder, Kerr Stuart - is a narrow gauge saddle tank engine who lives and works on the Skarloey Railway.

Peter Sam, named Stuart in his younger days while being brought up under the instruction of Duke, is extremely enthusiastic, eager and bubbly. He can be occasionally cheeky, but is kind-hearted, well-meaning and good mannered. He can also be somewhat prone to being a little naïve at times and perhaps take things a little too literally. With an infectious attitude and high spirits, Peter Sam is kind and hard-working to a fault and seldom grumbles about late hours and/or extra work. He accepts whatever job he gets and is always nice to the coaches and trucks, unlike Sir Handel.

Despite being friendly with everyone, when the bigger engines, such as Henry or Duck, tease him or cause him to misunderstand something, he can get very angry and wish to go crook on them.