Bertie (a.k.a. Bertie the Bus) was a failed Bio-Fusion experiment.
Not much is known about Bertie other than the fact that he was too ill to work. Because of that, he was put on display with other failed Bio-Fused experiments during the off-season that were also too unwell to work or were all already dead. Footage of him breaking down is shown in a scene where Thomas stops by a level crossing with Annie and Clarabel. Bertie's fate after the railway closed is unknown.