Interconnector capacity query

I came across this the other day:

http://www.dublincity.ie/YourCouncil/Documents/Taking_the_Dart_underground.pdf

Now.

On page 11, the presentation states:

High capacity - In excess of 28,000 passengers in both directions

Frequency - In excess of 20 trains per hour

The most realistic interpretation of the above capacity figures is that there will be 20 trains per hour in each direction, though it doesn't specifically state it.

This is sensible on the part of Irish Rail. Their original idea of signalling only for up to 16 trains per hour was rather unambitious - even working at that level it would leave the tunnel being used at just above half of its theoretical capacity (realistically, around 30 trains per hour in each direction).

However, in order to use the tunnel to this increased capacity, it is necessary to have appropriate arrangements at either end. (i.e. turnback facilities to take account of capacity restraints on the lines leading to the tunnel, particularly the Northern line).

Heuston appears to have such a turnback facility (extra platforms), as shown in the station layout diagrams in the above presentation.

However, the layout of the proposed Spencer Dock station and its environs (on pages 40 and 41 of the presentation) do not appear to include any turnback facility. Rather, there are only two platforms in the station and an apparently straightforward connection to the Northern line.

In the absence of a turnback facility in or around Spencer Dock, how are the proposed frequencies (and capacities) to be achieved, given that it would not be possible for all tunnel services to use the Northern line because of the presence of Arrow and Enterprise services.

It's possible, of course, that the diagrams used are old ones - the diagram on page 43, for example, looks a bit out of place - but it may also be that Irish Rail have some very clever plan.

Does anyone know?