Its purely a demand/distance function and there is an equation which relates the journey time, frequency and population of a town to the demand it will create which was devised for Dublin some years back, so as Dundalk has a smaller population and is further away than Drogheda its demand is lower
Further out a commuter train goes, the fewer it carries and the fare for longer journeys is less than that of a shorter journey. So the cost to run further increases quickly. Some evening trains to Drogheda manage to turnaround and get back to Dublin to provide another peak hour service
There are plans for additional services on the northern line and these will be focused on Dundalk/Drogheda with limited stops and will be intercity railcars. Probably looking at something happening in 2012 on this as the trains and new signaling should be in place
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