Is Zakat obligatory on taxi cabs or on the vehicles of transportation and freight companies?

There is no zakat on taxi or transport vehicles themselves. Zakat is only obligatory on the fare (income) if it reaches the nisab (minimum threshold) and a hawl (full lunar year) has passed over it. This is because these vehicles are prepared for use and transport, not for sale. However, if the vehicles are prepared for sale, then zakat is obligatory on them as trade goods, based on the hadith of Samurah ibn Jundub: "The Prophet, peace and blessings be upon him, used to command us to pay charity from what we prepare for sale."