It's a complete myth that it's impossible to build new diesel trains due to emissions standards. The class 800s will be brand new diesel trains, and the class 68s are still coming on-stream.
Correct.
What is true is that it is
difficult (but not impossible) to build Tier IV diesel-electric freight locomotives or multiple units
for the toy train loading gauge used in Britain. It all comes down to the question of how much you're prepared to pay for the engineering work involved in the packaging, and what compromises you're willing to accept.
The case you mention of the MTU gensets going under the floor of the Hitachi SET is an excellent example of this. To accommodate the new Tier IV compliant gensets and achieve the required performance, Hitachi were forced to build a humped floor where a version built for a market with less restrictive emission standards could have an older non-compliant genset underneath a flat floor.
There are unlikely to be any new GE locomotives exported to Britain any time soon, as the Tier IV compliant GEVO package cannot fit - it's even too tall for Australian mainline* usage. I'm also not aware of them having confirmed that the PowerHaul engine has been certified as Tier IV compliant, GE would only take that step if it was both possible (it might be like the GE 7FDL engine and the EMD two stroke engines - impossible) and if there was a profitable market for it.
Progress Rail Services (which absorbed EMD a few years ago) are also unlikely to be building a Tier IV compliant successor to the JT42CWR in the next few years as they do not yet have a full power (i.e. 3,300 kW) Tier IV loco for the domestic market in North America. The reality of business dictates that locos for export to the small UK/EU market will be a lesser priority than getting a full size loco ready for the much larger domestic market where they can sell many hundreds, even thousands if they do it right.
Complicating this issue with the industry's big players is the problem that there is not as much chance in the near future for a widespread modernisation program similar to that which resulted in the JT42CWR orders being large enough that they had an economy of scale approaching that of a domestic model. Without the opportunity to sell a good number of them, Progress Rail and GE are not likely to put in the R&D work to package a good powertrain into a UK-size body.
The result of all this is that UK operators will be paying well over the odds for any new diesel-electric locomotives for the foreseeable future, because they'll be at the mercy of buying bespoke designs from low-volume companies such as Vossloh which will use off-the-shelf prime movers or gensets from third-party suppliers such as Caterpillar, MTU and Cummins. These designs will get the job done, but they will not have any of the benefits to be found in a Progress Rail or GE design where there is the opportunity for internal collaboration (such as the work which went into repackaging the SD40-2 for UK use and for large-scale production to spread out the R&D costs.
* The Australian mainline loading gauge is larger than that of Britain, but still well short of full size. Bureaucracy has so far prevented the type approval of taller locos on the Parkes-Adelaide-Perth and Adelaide-Darwin corridors where they would still be lower than the double-stacked containers they haul! No such restriction applies to the isolated iron ore railways in the Pilbara region which are the world's heaviest freight railways.
