Increasing the capacity of the Northern Line and enabling it to cope when things go a bit wrong (as they inevitably do) may run into difficulties because 3rd or 4th tracks are very expensive. However by comparison fully banalised signalling all the way to Drogheda, plus a few sets of properly located crossovers should be relatively inexpensive (no heavy civil engineering). I realise that use of the"wrong" road may not be possible in all circumstances, but it should ensure that Enterprises could overtake stopping trains on occasion. It would also make for much easier maintenance, track posession, etc.
Squeezing maximum capacity out of existing lines should be a high priority throughout much of the system.
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