Would not the size of the filter depend on the engine output, so for a class 68 it 3 meters long?
Not so, the quote was for the 68. However, it does depend on engine size, the 88 has just two, mounted side by side on the roof.
The EuroDual is a different beast altogether, the link provided was for the earlier Vossloh design. Stadler have already built a EuroDual which is 7MW electric and 3MW diesel. It is larger than the PrasaDual but apart from the Co bogies similar body design to the PrasaDual which is only a Bo-Bo.
Weight distribution also plays a big part in any redesign. It has also been said that the 68 body, at 20.5 metres cannot be any longer or it would be outside the RA7 classification.
Regardless of the above, since the 66 cannot have it fitted either why would the 60 be able to when it has the same size body.
The filters not only have to be close to the engine, they also have to be easily accessible for routine maintenance, so if they cannot be accessed from the body sides there needs to be space for the fitters to work on them inside. I have not seen either the PrasaDuals or the EuroDual from above so cannot see where they are mounted but I suspect in the roof space
Stadler have produced a very large layout diagram of the 88, though nothing is identified, here:
https://wwwstadlerrailcom-live-01e9...8a9e-1b73cc96d894/leurodualukbrlldrs0816e.pdf At maximum zoom it is still good quality. As yet they have not published similar for the PrasaDual or the EuroDual but they may appear soon.