It was usually 'baulk road'. Each rail was bolted down to large longitudinal baulks of timber; the bolts alternating between each side of the bridge rail. The square-headed bolts are inserted from the top, passing through the foot of the rail, the packing pieces, the baulks, and then screwed into a metal 'fang shoe' which bites into the underneath of the baulk. At intervals the baulks are held apart be a timber transom. Alongside each transom is an iron tie-bar with nuts on each end holding the baulks tightly against the ends of the transom. On mixed-gauge track there will be three rails, mounted on three baulks, with two transoms, a long one between the common and narrow (standard) gauge baulks, and a short one between the narrow and broad gauge ones. The ends of the transoms are cut at an angle, and fit into angled notches cut into the sides of the baulks. The rails are not laid directly on the baulks, there are small cross-grained timber packing pieces placed between them.
Baulks road is still sometimes used, though obviously not broad gauge. The tracks in Paddington Station were still laid that way about ten years ago when I last looked; I don't know if they still are, or why it was done that way. Baulks road is also come times seen on bridges; I think that's done to reduce the weight.
If you look at photographs of broad gauge track you will see that the transoms are much more widely spaced than sleeps would be, but I can't remember what the distance was now.
I did some of the work on the broad-gauge track work at Didcon many years ago, mainly drilling 15/16 inch bolt holes in rail with an old fashioned ratchet drill which was found to be better for the task than a modern electric one. I also did some of the work on laying the edge slabs on the transfer shed platform.
