Sorry Valliant, my badly written English, gives the wrong impression. Your quite correct and technically a 14 foot water course is sufficient to pass two narrow boats neither I or anybody else would be that foolish to try. Canal builders would have used and reused time and again their timber formers to build the bridges, thus all bridges on a particular canal would have been practically identical.
That is how the half dozen barrel roof cottages on the Lower Stratford came to be, built using bridge formers and the two ends bricked up. They also have tie bars through them and abutments to the walls to stop them spreading outwards under the weight of the 13" solid brick roof.
It wasn't the waterway width of bridge hole that concerned me, those are well documented, more the required length of bridge on the waterway plane, that needs to be adjustable to suit the numerous different width road assets that are available in Trainz.
There is was no standard road widths in the UK, sizes were sometimes set out by the local council but seldom.
Asset creators have made dozens of roads of all various widths so these when connect to a bridge with an inbuilt road spline they either fail to meet the bridge side wall or stick out through the brickwork and overhang whatever is beneath.
The other point I pondered, canal bridges tend to be pinched inward at the centre.
No problem to do that and recreate on a model, but how to get a spline road to do so at that point?
My very limited asset creation experience, suggests to me that can't be achieved as spline roads would have the width specified in the config file presumably.
Anyway, not too bothered about the narrowing of the bridge roadway, parallel sides will do.
Maybe one answer is to specify the Kuids that I know will fit in a read me file.
Cheers
JP