If a country has a pattern of tourists being targeted by criminals on public transport, that information is mentioned in the Country Specific Information in the section about crime.
Taxis. Only take taxis clearly identified with official markings. Beware of unmarked cabs.
Trains. Well-organized, systematic robbery of passengers on trains along popular tourists routes is a problem. It is more common at night and especially on overnight trains.
If you see your way being blocked by a stranger and another person is very close to you from behind, move away. This can happen in the corridor of the train or on the platform or station.
Do not accept food or drink from strangers. Criminals have been known to drug food or drink offered to passengers. Criminals may also spray sleeping gas in train compartments. Where possible, lock your compartment. If it cannot be locked securely, take turns sleeping in shifts with your traveling companions. If that is not possible, stay awake. If you must sleep unprotected, tie down your luggage and secure your valuables to the extent possible.
Do not be afraid to alert authorities if you feel threatened in any way. Extra police are often assigned to ride trains on routes where crime is a serious problem.
Buses. The same type of criminal activity found on trains can be found on public buses on popular tourist routes. For example, tourists have been drugged and robbed while sleeping on buses or in bus stations. In some countries, whole busloads of passengers have been held up and robbed by gangs of bandits.
Tuk-tuks. In Thailand, there are 3-wheeled vehicles called tuk-tuks mainly because of the sound they make. Most will hang around in tourist areas and offer to take you to massage parlors or for a "free" tour. They get a cut of anywhere they take you whether it be a jewelry store, bar, or massage parlor. Upcountry, they are mainly legitimate. The ones I am talking about are mainly in Bangkok.
Baht Bus. Not really a bus, but a pick-up truck with seats in the back. They are covered and are used primarily in Pattaya, Thailand.