Behind the Scenes
Series 5 Model
Bertram's model was custom built to run on O gauge track to the Gauge 1 Scale Standard. The model had a motor to power it mounted inside the chassis as well as an eye mechanism. The eye mechanism used servos which were mounted in the cab, the servos were hidden by blacking out the cabs and windows. Metal rods went all the way from the servos in the cab to a bracket in the smoke box behind the faceplate, one for up and down movement and one for left and right movement. This limited the range of movement of the eyes as well as being cumbersome and jamming often. Bertram's side rods were sourced from a OO scale Triang Hall class with little modification. Bertram's wheels were sourced from Slater's 10 Spoke Wantage Tramway driving wheels and 10 Spoke LNER 02 Bogie Wheels for the tender. These wheels were used on an O gauge locomotive chassis..
According to Britt Allcroft, Bertram was supposed to be a tank engine. However, Duke's model and Smudger's face-mask were recycled to create Bertram as due to financial difficulties. After the events of the fifth season, Bertram's model was later reverted back to Duke. Bertram was only ever depicted with one happy face-mask. The face was first sculpted in clay, and from that resin casts were made of a silicone mold.
Large Scale Model
For ease of filming, Season 5 introduced larger-scale versions of the narrow gauge engines, and from Season 6-12 these larger models were used almost exclusively. Due to budgeted reasons, no large scale model of Bertram was built, this can be attributed as to why he never appeared after the fifth season.