In 1977, the trams were already equipped with bus indications, and it was known that this line did not have a long life ahead of it: it was finally to be closed down on 31 July 1978.
That it was (together with the route to Wemmel) the last remaining one, is no coincidence and this can be seen in the photo: the 3 trailers and accompanying conductors were really not used to transport air, although the photo - without passengers or staff - suggests otherwise.
Only passengers with a season ticket were allowed to board the motor car: the sign '1 agent' indicates this.
It was probably ready to depart for Brussels, to bring the passengers back home in the late afternoon.
In any case, a beautiful postcard!

Thanks for your description. Is it possible to pinpoint the location on the map?

Done, and as bonus: https://goo.gl/maps/SFVuc6Rpwd7Qzoce6