Abstract: An undercarriage for heavy diagnostic equipment having a selectably engageable swivel mechanism activated by the movement of a cam to allow the undercarriage to be turned when desired and rolled in a straight line when necessary, a selectably engageable braking mechanism to prevent movement of the undercarriage when desired, but allowing the undercarriage to be moved quickly if needed, and a balance wheel assembly to prevent the undercarriage and equipment from becoming unstable when encountering obstacles, while allowing the undercarriage to be rolled up inclines. An aspect of the invention allows the swivel and braking mechanisms to be selectively engaged from the same point along the undercarriage, thereby providing superior control of the undercarriage and the equipment which it carries.