I went and pulled mine off the shelf and I forgot to put the engineer back. This shows that the control stand is attached to the upper boiler backhead and when sliding the shell back to clear- the engineer's legs get smashed between the control stand and the diecast seat stand. Removing the figure allows the cab to slide back, clear the little external lip. The engineer was glued to the diecast seat.
Again, in this first picture, the cab must slide backwards but the entire boiler backhead and the attached control stand then moves back with the cab towards the seat that is attached to the floor and lower frame.
This side view externally, the little red ledge or lip on the bottom of the cab has to slide back to clear the lip of the walkway on the side of the boiler (attached to the lower frame). Again, the entire boiler shell slides slightly back, taking the cab with it while the walkway remains stationary- but that engineer's legs prevent the shell from coming all the way back.