The smaller UK loading gauge means most continental designs need scaling down, and smaller manufacturers won't put the investment in for a handful of sales.
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Except Stadler is one of the companies that does exactly this - bespoke business.
Stadler is a Swiss company, and over time it has built a lot of small batches of different types for different Alpine metre gauge routes where the Alpine loading gauge is very very similar to UK main line loading gauge.
http://joycewhitchurch.zxq.net/gauges/text.htm
contains a RhB loading gauge diagram, I know that as I sent it to the site owner to use, and RhB is one of the larger sized metre gauge routes.
I suspect there are other commercial reasons Stadler does not seem to bid in the UK market, some companies deliberately keep out of something they see as too big a risk.
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Nick