Indicating a ‘flattened` semicircular arch, used to keep height reasonable, to reduce approach gradients, and to increase the width of gauge clearance below.
An arch based on the ellipse form; 
(Learning Modules / Mathematics / Bridges) The exact shape of an arch may vary hugely. The simplest is the semi-circular, but a shallow, flattened semi-circle becomes an ellipse, and many mediaeval stone bridges were built with elliptical arches spanning rivers.
One of the strongest arches in brick masonry. It springs from a horizontal seat at and on the spring course, and the way its haunch crowns up adds to its strength