When connecting to the main line tracks after Deighton all trains go under down through the tunnel. However there’s another junction which can be used for fast trains avoiding the tunnel making it more efficient for fast trains and could reduce traffic on the 2 tracks used.
It's actually not more efficient as to get onto that required track at Ravensthorpe from Dewsbury, you have to slow down to 20mph for the crossover instead of just slowing down to 45mph to go via the tunnel route. Plus you're held at the lower speed for longer due to the position of the crossovers.
Looking at my 1988 rail atlas it's shown as a freight only chord presumably for Healey Mills traffic mainly.
