Actually, Brunel had a good reason for the wide gauge. It allowed for a wider (and therefore more powerful) firebox between the main drivers of the locomotives, making it possible to lower the center of gravity.

If I remember correctly reading that Brunel's tracks were also more stiff than the "normal" tracks. He kind of did things which we now see in rather modern ballastless tracks. Of course, it did require more precision in the trackbed.

From the technical point of view, Brunel's wide gauge was superior to the standard gauge at its time.