Sound Fitting A Heljan Railbus
There's a few variatons of the Heljan Railbus so the method shown below does vary slightly, but they all come apart in a similar way.
Start by removing the Horns and hadrails.
Then prise the body upwards using a plastic tool so that you don't damage the paint.
The decoder is under the pcb on top of the model, you need to remove two screws from the black plastic part which holds it to lift it off.
Make sure no plugs come out of the board as they come out fairly easily. Place the speaker in the space between the doors, facing upwards.
If you need sound for a railbus find it here.