The Park Royal Railbus kit is more prototypical than I had expected. Now I've actually done some research I find that the power unit was a separate item to which the body was bolted. Quite sensible really as a simple lift would remove the later, leaving the mechanical gubbins available for spannering.
The brass kit chassis is encased in plastic parts. I’d wondered what the holes in the brass either side of the motor and toward the back end were, and it turns out that some plastic pillars go through them and into the body. This allows for realistic separation between the two parts and also enough space for the top of the motor and pick-up strips.
If I was being picky, the plastic details aren’t as sharp as I might like but they are good enough for a working model. If you want to go to town there are definite possibilities to improve matters a little, for example the springs on the end (dampers I suspect) could be chopped out and replace with sections of bolt or even real spring to improve definition. The pictures I have found are unclear but I’m betting that there was a certain amount of pipe and cable work that Airfix ignored for production reasons.
In the original kit the chassis is simply glued in place. On a working model this would preclude maintenance so a single bolt fixing is allowed for. These original pillars will still be used for location but must now remain glue free.