Well spotted, sladys! I'm surprised this unique piece of railway history has not been found before on GE.
I lived no more than three miles from the arch from 1965 thru 1973 and never knew it was there!!
Your link http://www.dmm.org.uk/archives/a_caus01.htm makes interesting reading. Did you note there two building costs; £1200 and £12000? Methinks £12000 would have bought County Durham in 1726!!