The 'Sleeper Spline' is used to fill in the space between two single deck plate girder bridges. On my routes, I use almost exclusively the 'Plate Girder Bridge, bendy' by maddy25, kuid2:124017:30014:1, mainly because it is curve able, or 'bendy'.
When placing two or more of these bridges side by side, set the 'Sleeper Spline' elevation just under the rails of the bridge. Add splines points to adjust for the curve of the bridge.