i think it might be inattention to the thicknesses.
previously, i asked the guy who has the range now to for a quote on what it might cost to upsize a Z scale B&O wagontop boxcar.
another of the hypothetical TT freight yard's 'exceptions' ... in the absence of common prototypes i see TT layouts being full of oddities like wet rock cars, hauled by things like DD40xs and KMs (!) unless you're modelling canada in which case your garden variety needs are far better catered for! anyway, another topic.
the guy went ahead and upsized the car, but as i recall it was going to cost c. 40 euro. i'd only asked for a quote on what it
might cost to do it rather than doing the work ... although i don't know that there was a whole lot of work involved. in any event he didn't seem grumpy that i didn't buy it. i imagine the z scale thickness was just inflated to TT and it used far too much material.
shapeways has potential for flat sides, ends for composite boxcars, hoppers and gons ... the roughness of their idiosyncratic -read slapdash- printing process would be better at rendering wood texture rather better than steel sheet. in the UK at the moment, shapeways is starting to hit the hobby mainstream, but a lot of the designers are kind of witless ... in love with an RTR dream that's never going to happen via the even-a-stopped-clock's-right-twice-a-day attitude of shapeways quality control. i saw an RS2 or 3 in N at a show recently done by their process; the guy selling it had the 'anything's possible, sky's the limit' kind of wild-eyed look about him as he showed it off, but frankly, on close inspection it looked like it had been carved out of pumice stone.
however, on composite sides ... think of how rough the accurail H0 composite boxcar is ... people think that's a
great model, though the rough texture is out of all proportion to what a real car would have. i have no clue why ... perception of what wood
ought to look like, i suppose?