Ribblehead Viaduct

[Signal Head]

I recall an explanation from a senior railway Civil Engineer for the singling, and specifically centralising the track, was that it had been worked out that the most likely failure point was the spandrel walls, so moving the track away from the edge both reduced the likelihood of that occurrence and it's consequences. "The intention is to prevent a train exiting the viaduct through the spandrels" was the phrase used I think.
It wasn't, as some people suggested at the time, to prevent two trains being on the structure at the same time, as that could have been achieved by a bit of extra signalling/interlocking.

 

[nimbus21]

Bulging spandrel walls are a common problem on arched bridges as evidenced by the remwdial tie bars and circular patress plates visible on many structures.