I am a great promoter of diversity: variety is one of the spices of life while homogeneity is soooo boring. For my money, fenugreek (Trigonella foenum-graecum) is the other spice which really gives life a lift. A while ago I mentioned that I'd travelled from Paris to Lisboa by train in 1989 via Hendaye & Irún. Why did I choose to single out this pair of border way-stations rather than the great cities of Poitiers, Bordeaux, Vallodolid, Salamanca through which I rattled on my 24 hr transcontinental journey? The answer is in the word 'border'. There are a lot changes when you cross the River Bidasoa from Pyrénées-Atlantiques to Gipuzkoa. The majority of the people on both sides of the border are ethnically Basques, or Euskaldunak as they would call themselves. But the language of administration changes at the border, as does the width of the railway lines. In 1989, that still required the lifting of each carriage from its French (standard gauge 1435mm) bogies to slightly wider "ancho / trocha / botola ibérico" at 1668mm. The latter is a compromise between 5 pés portugueses 1664mm ~= 6 pies españoles 1672mm. You could insist on a narrower tolerance if it wasn't for the fact that railways go round curves and bogies typically have 4 wheels on two axles. It was decided early on that it was more efficient to transfer the whole carriage than tranship the passengers and their baskets of chickens, hat-boxes, umbrellas and small children. And this has to be true for freight which would not only take time to unpack and reload (this is 100 years before the shipping container was thought of) but would also suffer 'shrinkage' - the euphemism for breakage and pilfering by the stevedores.
Accordingly, the passengers were all asked to leave the train which was stopped carriage by carriage in a special shed for exchanging axels. Things have moved on since then with the express-rail TGV in direct competition with medium-haul planes, you cannot have a leisurely piffling about in the Basquelands. This video will make crystal clear the high-tech system for cambiador de talgo especially if (maybe only if) you understand Spanish. I tell ya: hats off to engineers!
A destructive test of the allowable tolerance between gauges occurred on 18th December 1867 to a moving train just outside Angola, New York. The rail system of North America had not yet entirely rowed in behind Stephenson's standard gauge of 4ft8.5in / 1435mm and in that part of the North-Eastern US, the railway companies would run 'compromise cars' which could travel on both standard and 'Ohio Gauge' 4ft10in / 1473mm track. The gap is significantly wider than between Portugal and Spain. That was okay but there was a little lateral 'play' in the wheels particularly when travelling on the wider gauge and at certain speeds this would build up resonance so that each lurch would be a tiny bit more. This is a well-known problem in engineering; requiring soldiers to break-step when crossing bridges. The last carriage of the Angola train rattled right off the track and plunged down an icy embankment. The passengers were unceremoniously dumped onto a red-hot solid-fuel stove at the lower end of the car, the other stove unseated itself from the far end and showered its load of smoking coals down on the heap of humanity. The wooden carriage was soon a furnace. It took five minutes for the screaming to stop and probably 50 people died. The Angola Horror gave great impetus to standardisation of rail-gauge across the United States.