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A neat image from Wikipedia:

railway carriage side looks neat

Here the railway carriage side looks like it is crafted from a single sheet of metal (if we ignore the doors and the windows and some minor parts like that small thing near the carriage number) - no seams, no rivets, no anything just a 20+ meters wide and about 3 meters tall single sheet of metal.

Is it indeed possible and reasonable to craft a carriage side of such large single sheet or should I expect that there're neatly hidden seams somewhere?

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  • $\begingroup$ You don't see the individal sheets of drywal in a long hallway either, but that's still how the wall was constructed. It should be no surprise that there are techniques for hiding seams of things made from smaller panels. $\endgroup$ – Olin Lathrop May 21 '15 at 12:49
  • $\begingroup$ @OlinLathrop Well, we know two things for sure - drywall cannot be produced large enough to cover the whole hallway wall and if it could be then it would be next to impossible to handle it. $\endgroup$ – sharptooth May 21 '15 at 13:11
  • $\begingroup$ The point is that just because something looks like it is created from a single sheet doesn't mean it is. I used drywall as a common existance proof. $\endgroup$ – Olin Lathrop May 21 '15 at 13:17
  • $\begingroup$ That specific coach is a former British Rail Mark 3A, which might help with Googling. $\endgroup$ – David Richerby May 21 '15 at 14:16
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    $\begingroup$ @ratchetfreak I don't know if that means that each side was made from a single sheet or not. "All-welded" could mean that the side is a single sheet, welded to the chassis, roof and ends, or it could mean that the sides are made from multiple sheets welded to each other, to the chassis, roof and ends. $\endgroup$ – David Richerby May 21 '15 at 18:36
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Is it indeed possible

Yes

and reasonable to craft a carriage side of such large single sheet

It can be reasonable, but typically isn't. To form an entire side of the carriage from one sheet of steel would require a forming press that is gargantuan. However, one could instead have three more reasonably sized presses and then weld the sections together. Further, this could be more flexible - changing out the press plates is easier and faster, storing smaller press plates is cheaper and easier, if the machine breaks down you can still use the other presses, whereas if one piece of the gargantuan press breaks down, the entire operation might be waiting for the repair.

There are a whole host of issues that follow the same reasoning - if a panel is damaged, it's expensive and time consuming to rework, and affects a much larger part of the carriage than a single damaged panel would. The pieces are much harder to work with, move, and fasten, etc, etc.

So it's very unlikely that it's a single seamless sheet of steel or aluminum.

or should I expect that there're neatly hidden seams somewhere?

You should expect seams, and in fact, as ratchet freak pointed out:

"The bodyshell is [...]of full monocoque construction with an all-welded mild steel stressed skin,"

Monocoque is a technique where the skin or surface of the construction serves an integral support purpose.

The seams, therefore, may also provide support as ribs, and so you may find that having seams provides an added benefit, or perhaps reduces the need for internal supports.

The seams, of course, are not visible for both aerodynamic and visual appeal purposes, and it's relatively easy to hide weld lines with grinding, sanding, and polishing.

So without actual proof of manufacturing process, I believe you can safely assume smaller sheets with invisible seams are more likely than single seamless formed sheets.

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It's possible, aluminum sheeting comes in large rolls; You take one that is wide enough and you can cut out a single sheet long enough for the length of the carriage.

It's probably attached to the structure using welds on the inside

However seams can be very well hidden under the paint after welding them together and sanding it down.

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