Fan-Fiction Characters is a fan-fiction thing.
Adam is a warehouse engine saved from scrap by Sir Handel. He is introduced in "Sir Handel To The Rescue". He has the same model as Wilbert and has the same face as Culdee. He is based on Hunslet WD 0-6-0ST like Wilbert and Sixteen.
Arnold is a stuck-up diesel introduced in the episode "Arnold". He sucks the Inspector's hat and breaks down before his train. Adam and Cartman take the express. Arnold creeps away leaving a nasty smell and a battered bowler hat. He is based on Class 56.
Weasel is a gray Class 08 diesel engine. He is introduced in "Pop Goes The Weasel". Weasel tried to send Percy away by telling lies about him. The Fat Controller sends Weasel away.
Stewie is a high-speed Narrow Gauge diesel. He is introduced in "Duncan & Stewie". Stewie runs out of diesel oil in Duncan's Hill. Duncan rescues him. He is based on the Talyyin Railway high-speed diesel.
Wallace is a Great Western steam engine. He is introduced in "Donald To The Rescue". He has a breakvan named Frog. Donald saved them from scrap. Once he was repaired, he worked with Cartman on Cartman's branch line.