The latest Narrow Gauge Society magazine has an article about the steam locos built for the Gibraltar Dockyard construction lines. These are metre gauge. Mention of tipper and flat wagons but nothing about box wagon. Article is by Alan C Baker, who has a big interest in these lines.
These contractor railways were apparently used to build various instalations including some in Singapore. Some also worked in UK on 3ft 3in gauge as this was close enough to metre gauge.
I should listen to that video though.
That confirms it is metre gauge, but it seems the system was not just for construction. Probably explains why the contractor's line was metre gauge though.
Narrow gauge Francophile interested in 1m, 60cm,50cm , 40cm and smaller gauges . Build in scales from 1/6th to 1/24th. Also 1/32nd and 1/35th using 16.5mm track to represent 50cm and 60cm gauges.