This question calls to mind a thread on the so-called 'Berne Gauge' of some ten or twelve years ago either here or on Usenet. I made some notes at the time which I use here but I'm afraid that I no longer have a record of links to the original threads with the exception of a reference to a
Modern Railways article sometime in 1992.
The term 'Berne Gauge' was/is often used to refer to the loading gauge of continental rolling stock but itself was a simplification of the overall situation. The term itself arose from the fact that a conference was held in that city in 1913 which tried to improve on the loading gauge then generally accepted for vehicles in international traffic, without the need for each individual design of vehicle to be submitted to every railway over whose lines it was proposed to run. The original (1891) gauge meant that everyone accepted stock built to the then standard French loading gauge, which was the smallest in mainland Europe as to both width and height. All the other railways had very similar width requirements, and the new gauge was only some 50mm wider.
However the new gauge raised the height by some 130mm, as an attempt to compromise between the French standard and the generally more liberal heights available elsewhere. But it took quite a long time before all French main lines conformed to it, particularly in the west of the country, and full clearance was not achieved until the late 1930s.The gauge thus defined continued to be known officially as the
Gabarit passe-partout international, or 'PPI', which translates literally as 'pass everywhere international gauge'. The convention which brought it into use was signed in Berne at the end of the conference mentioned above, hence the unofficial name Berne Convention gauge, shortened to Berne gauge and it came into force on 1 January 1914.
Latterly, long after the the formation of the UIC (International Union of Railways), of which the four main-line railways of Great Britain were members, UIC A gauge was formulated, marginally higher than the PPI gauge. A slightly higher gauge, but still lower than that available in Belgium and Switzerland became UIC B gauge, while the 'Central European' gauge became the UIC C gauge. This is an over-simplification of the process, but is accurate enough for this purpose! All these gauges had identical widths, which stretched from the Spanish to the Soviet or Russian border, except for Scandanavia.
For passenger traffic, the width difference is also relatively unimportant (although for sleeping cars only, the height difference is not; one cannot get three berths one above the other within the BR loading gauge). This is because the way that the UIC gauges are defined creates a far greater reduction in permissible width with increasing length than is initially the case with the BR gauge. Thus while the short-wheelbase four-wheeler goods wagon of yesteryear would be a good foot wider on the mainland than here, the standard 26m-long UIC passenger coach has to be no more than 2,825mm wide. For the imperially minded that is 9ft 3.5in, which is (surprise) exactly the same overall width over door handles as a BR Mk 1 coach.
Where the UIC gauges and the BR gauges mostly differ is in the width allowed below the solebar, continental railways historically had very low platforms so the stock could keep its width practically down to the rail whereas the British railways tended to have higher platforms so limiting the stock width. With the spread of cast iron and steel as a bridge material instead of masonry longitudinal trusses rising up between the tracks were used in many places taking advantage of the reduced width of the rolling stock.
So the answer to the question is in two parts:
- To permit unrestricted use in the UK of even the smallest UIC loading gauge both these structure gauge constraints will have to be removed
or
- if the British structure gauge(s) are to be retained determine whether the the coach body can be reworked to allow the narrowing under the solebar and whether the under-floor equipment can be rearranged to fit the narrower space.
Both cost money!
