One would have thought that a priority for rolling stock designers would have been to make the best use of the limited UK loading gauge. Seemingly not. The illustration of the interior of the new locomotive-hauled Nova 3 coaches for Transpennine Express is taken from an article in International Railway Journal; I hope this is acceptable under the fair use of copyright rules.
Take a look at the skirting area. This shows the problem caused by the very sharp lower bodyside curvature apparent in exterior views of the stock. This example is almost the rule. The same thing affects much of the rolling stock built since 1990, including the BR-designed Networkers and the BREL Electrostars and Turbostars. The bodyside profile makes no sense within the parameters of the UK loading gauge.
As is well known, the British loading gauge is little bigger than that permitted for narrow gauge railways such as those of Japan and South Africa. This is mostly due to the closeness of adjacent tracks and the low bridges and tunnels, but the problem is aggravated by the British practice of having high platforms, approximately 90 cm high. The issue was carefully examined when British Railways was formed in 1948, and the result was the C1 loading gauge (diagram above), which applied to passenger vehicles of a nominal width of 20 metres with bogie centres 14.17 metres apart - as close as practical to the vehicle ends. The mark 1 stock was built to comply with this standard, and more recent stock such as the Bombardier class 377 Electrostars are constructed to the same main dimensions.
Eventually, longer vehicles came into use. These were made narrower, in accordance with a geometrical formula to take account of overthrow on curves. Another important change was the use of air springing which is softer, which meant that carriages had to be narrower at the cantrail, where the sides meet the roof.
As far as the passenger space is concerned, the salient point is that the maximum width of the loading gauge is available from 1.225 metres above rail level and upwards for about 1.2 metres, when the vehicle body has to become narrower. The C1 loading gauge for 20 metre vehicles allows a full 2.82 metres, tapering to 2.62 metres at the cantrail. Logically, it generates a profile similar to that in these vehicles below.
The important factor here is the floor height. The standard height used to be 1.3 metres, which resulted in no loss in width, leaving space for skirting level ducts without cutting into legroom; this is a feature of, for instance, all the BR mark 1 stock. The Hitachi 800 series are also unaffected due to the high floor level enforced due to the size of the underfloor engines. Suburban stock, on the other hand, with a lower standard floor height, can suffer badly from reduced floor width, although it does not affect the Siemens Desiro class 450 and similar types. That said, one wonders why lower bodyside curvature is needed on suburban stock at all as the footsteps project beyond the bodysides.
The problem can be seen clearly in the top photograph; passengers sitting next to the windows will only be able to put one foot on the floor unless they twist themselves at an angle. Why, then, do these vehicles have such pronounced lower bodyside curvature, since there is no necessity for it through loading gauge constraints?
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