Tom Tee -- I don't think you need to cut the rail on the diagonal - certainly not at the hinge end when the hinge point is above the rail top, as the first movement of the rail on the bridge is 'out and up' away from the fixed rail at the abutment. At the opposite end, due to the raised hinge point, there is a slight movement that closes the gap between the fixed and moveable rail, but not by much before it 'clears'. Consider the geometry, what one has in cross-section is a rectangle whose length is the length of the bridge/moveable rail (in this case, 48 inches) and whose height is defined by the thickness of the bridge plus roadbed plus rail. The clearance distance between the two fixed rail ends is defined by the length of the diagonal of that rectangle. Suppose the total height of the bridge bottom, roadbed and rail is 1 inch, so whipping out my old high school geometry, the length of that diagonal is 48.01 inches (you can do it either with Pythagoras or tangents and cosines) - so that means the gap is 0.01 inches (all of this is based on having the hinge point at the top of the railhead = top corner of the rectangle). As a practical matter, a 1/32 gap would cover this requirement. Better would be to have the rail ends on the lift side of the bridge slightly proud of the rest of the roadbed, etc. - say ~1/8 inch.
This reminds me that in fact there is a fourth feature to consider, which is the stability of the fixed distance between the two rail ends on either side of the bridge. That needs to be pretty stable. Some of the designs/pictures of moveable bridges shown on the various threads here show a connection between the two table sides at the bottom - which means a 'step across' - to maintain that distance
