This is a 1:32 plastic kit of the well known Harrington Legionnaire coach on a Bedford VAL chassis from the Italian Job film.
There are full assembly instructions here: http://www.chase-cars.com/bodyshells/1964-italian-coach/ and the kit comprises of
To get it to fit on a Scalextric track and still fit 3 Minis makes it a big vehicle, 340mm long and almost 80mm wide. The structure is a resin bodyshell with removable floor, this accommodates the steering systems, motor pod and of course give somewhere for the Minis to ride.
The two stage gear reduction system gives it plenty of torque to drive around the track even when carrying 3 Minis, you can see it in action here
https://youtu.be/fLwn8VURqUs
https://youtu.be/lYEU9_N64kw