Comng back to this conversation after a couple of years, I came to the conclusion (well, pulled out my arse) that about 6' across would be the maximum practicable load that could be run at any speed on 2'/600mm track. This being three times the track gauge. ISO containers being 8' across would have to be run slowly, on good quality permanent way, e.g. the Welsh Highland line. Also, there's no guarentee the shipper would have loaded the things evenly.
However, as if to shoot this down, here's some ISO boxes on flatcars running on what appears to be a sugar cane railway in Queensland. With the proviso that one appears to be on departmental service, and the other a storage box that doesn't leave the railway.
https://model-railroad-hobbyist.com/node/25730
https://maroochyshiretramways.wordpress ... sel-fleet/
Following up, here's some snippets frm Wikipedia:
Festiniog Railway
Main article: Festiniog Railway
gauge = 597 mm (1 ft 11 1⁄2 in)
width (brakevan mirrors) = 6 feet 10 inches (2.08 m).[51]
width (brakevan body) = 6 feet 0 inches (1.83 m).
height = 5 feet 7.5 inches (1.715 m).
length = (carriage) 36 feet 0 inches (10.97 m).[52]
Lynton and Barnstaple Railway
Main article: Lynton and Barnstaple Railway
gauge = 597 mm (1 ft 11 1⁄2 in)
Lyn locomotive over headstocks
length = 23 ft 6 in (7.16 m)
width = 7 ft 2 in (2.18 m)
height = 8 ft 11 in (2.72 m)
Passenger
length = 39 ft 6 in (12.04 m)
width = 6 ft (1.83 m) wide,
width over steps = 7 ft 4 in (2.24 m)
height = 8 ft 7 in (2.62 m)
several standard gauge 73 class locomotives of the NSWR, which are 9 feet 3 inches (2.82 m) wide, have been converted for use on 610 mm (2 ft) cane tramways, where there are no narrow bridges, tunnels or track centres to cause trouble.