The first double track laid for the Liverpool & Manchester had the running lines spaced so that the inner rails were 4' 8" apart (the odd half inch came later) so that trains could run down the centre to allow for over-wide loads. The history of the L&M doesn't relate if this facility was ever used, and how a train was going to be placed on the centre pair of rails.
It would have needed some very fancy pointwork, or more likely some kind of traverser. But I suspect it never got used when the operators realised that it would have virtually stopped everything else on the railway. 
For a G1 double track line with 10ft radius curves, what is the recommended track separation?

