Katha-1-3-3-ātmānam̐

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

Here a chariot is imagined for the âtman, conditioned in Samsâra, entitled to acquire knowledge and perform Karma for attaining emancipation and for travelling in Samsâra, as a means to reach both. Know the âtman, who is the enjoyer of the fruits of Karma and is in the bondage of Samsâra, to be the lord of the chariot. Know the body to be verily the chariot, because like a chariot the body is drawn by the senses occupying the place of horses. Know also the intelligence to be the driver, furnished with the capacity for determination, because the body is mainly guided by the intelligence, as the chariot is mainly guided by the driver; for, everything done by the body is generally done by the intelligence. Know the mind with its characteristics of volition, doubt, etc., to be the reins; for, the senses, such as the ear, perform their functions when grasped by the mind as horses by the reins.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda