The Bachmann Class 20 is a well liked model with good traction & excellent pulling power (just like the real thing!).
To dismantle the model, first remove the two screws holding the body to the chassis, one screw under each buffer beam.
The body can now be gently eased away from the chassis. The cab section is a separate moulding to the main body assembly, and is held in two vertical slots each side of the body. Very gently ease the cab sides out and up past the clips on each side. As far as I can remember, I recall mine being tacked in place with plastic cement. You should now be able to reach the plastic glazing.