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In theory, yes, but it's not that simple. If you have a market-led model, and you carry on road-building away from the railways, as this Government seemingly still intends to do, you may end up with a less used older line and more motor traffic on parallel roads, leading to fewer trains. As above, it's not just about the simple measure of 'capacity' but about different qualities that a network can possess, and how different networks influence each other and interact. It might happen as you say in some areas, but in others the effect might be affected by extraneous factors.
Adding HS2 mean the regional line won’t be as busy allowing more trains on the existing line.