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Summit Cutting (behind the pumped storage power station at Tanygrisiau, opened 1963) is the highest point of the continuous gradient from Boston Lodge, and therefore the highest point from which any gravity run could be made. Beyond this point the line drops at approx 1 in 75 down to the original formation at Tanygrisiau.
Notice the additional rails in this final picture, laid outside the normal running rails. These were installed to satisfy the CEGB, as a precaution against derailments breaching the high pressure penstocks pipes that run below the railway just here and to prevent a derailed train going into the power station compound and getting mixed up with high voltage electricity.
As the line leaves the north end of the cutting, it crosses a private (Electricity Board) road which is known as Valvehouse Crossing. At one time it was known as Penstocks Road Crossing.