Unfortunately can't locate your tree to check out exactly who these William Alexanders are. My great-grandfather was also a William Alexander, the renowned builder of railway bridges (starting near his place of birth, Gourdon) and buildings, but... he was born in 1837 and not in 1854. He had several sons, one of whom was my grandfather, George Alexander (b Inverness, 1880). A son called William was born in 1872 in Gourdon and he had 3 children: William Noble Alexander, born in 1907 in Inverness, Alexander Noble Alexander (b. 1911, Inverness) and John Moir Alexander (b. 1913, Inverness).
William, b. 1872, was a stone-mason and grew up in the building industry too. However, when William married (Ann Noble, 1905), he took up farming on the Milton of Culloden farm. In 1914 he moved to Belmaduthy farm on the Black Isle.
Would really love to know who 'your' William Alexander was. I hadn't been aware that there was someone of the same name who was also a railroad- and bridge-builder.