Where Thailand is at its narrowest, pinched by Burma to a few miles' width just above the Isthmus of Kra, half a day's drive south of Bangkok, there is a pass through the hills with a remarkable history. Easy routes across the ‘dorsal spine’ dividing Thailand and Burma are few. Those further north (Three Pagodas Pass most famously) have for centuries seen armies invading Burma from Siam or vice versa, intent on sacking capitals and rocking dynasties but not greatly interested in the hill peoples in between except as guides, porters and spies. But the southern pass of Maw Daung has more usually been a trade route – and a way of escape.

From the train, the Bangkok–Butterworth ‘International Express’ which is itself a favourite vehicle of young escapers, smugglers and myself, the landscape looks innocuous, even rather dull – low hills tufted with light scrub and half-hearted secondary forest. Stumpy, rotten-tooth rock formations add a little variety but, as the train runs south, even these peter out and it is difficult to imagine the low ridges of the hinterland as being an obstacle to anything very much. But they are.