Originally the Channel Tunnel safety authorities specified that only single set trains with power cars at either end could use the Tunnel.
In addition, a complete train set (like those 18-coach units operated by Eurostar today) must be designed to be split inter two 9-coach half-sets should an emergency arise within the Tunnel.
The idea is that passengers would be evacuated from the set with the fault, through to the unaffected set. I believe this can be done on the level (ie without having to disembark). The functioning set would then be driven out of the Tunnel under its own power.
But these rules were laid down, probably, in the late 1980s and some of today’s high-speed trains have different technical specifications as noted by Drumochter.
The current Eurostar trains have power cars at either end. But the news ones, currently being built by Siemens, have powered-bogies although they remain single set trains.
The advantage of the latter is that you get better traction and the train will still operate even if one or two powered bogies fail (because the remainder of the powered bogies can still move the trainset).
In addition, train firms like them because they have lower operating costs. The space occupied by the power cars is no longer wasted.
For example, an existing Eurostar accommodates 750 passengers but the new Eurostar trains (with powered bogies) will accommodate 900. So that’s an extra 150 passengers for every train.
It appears the Channel Tunnel safety body has come round to accepting powered bogies (I say that because otherwise Eurostar would not have gone ahead and ordered a new fleet of trains) but the issue with DB’s twin-set ICEs is that, were an evacuation needed inside the Tunnel, it would be more difficult to transfer passengers from the faulty set to the functioning set.
As I see it, passengers would have to climb down from the train to ground level, walk along inside the Tunnel and then climb up into the other set.
The point is that not everyone of fit and able and many passengers, especially holidaymakers, will have luggage.
But on the other hand, and perhaps someone can correct me if I’m wrong, although there have been a few well publicised problems with Eurostar trains within the Tunnel, so far Eurostar has never had to split trains and evacuate passengers.
So that is why DB’s ICEs still await Tunnel approval.
I hope you can make sense of this ! I’ve tried my best to explain things as clearly as possible.