Small hint for Advanced Engineering

The engineers who designed the SRBs might have preferred to make them a bit fatter, but the SRBs had to be shipped by train from the factory to the launch site. The railroad from the factory runs through a tunnel in the mountains. The SRBs had to fit through that tunnel.

The tunnel is slightly wider than a railroad track, and the railroad track is about as wide as two horses' behinds.

So a major design feature of what is arguably the world's most advanced transportation system was originally determined by the width of a horse's arse.