Depending on your destination and why you are traveling there is a wide range of photo ops to look out for. There have been occasions where I would have preferred to have another more independent mode of transport than a train or a taxi. But even these methods can provide opportunities for a keen photographer. Its true you don't have the autonomy and can't just pull over at will. It does however free you from having to 'watch the road' or pay attention otherwise.
The following images are from travel in India. They were taken either from a taxi window or shot from a train.
The above shot and the following were taken through a taxi window using a 50mm lens. A lot of life goes on in the streets. The shot below though appearing posed was taken before I was noticed. The instant he saw me, he moved away. Most people don't mind too much having their photo taken but are a bit shy and become more self-conscious when they see the camera.
Just briefly, For most of the street photography that I do the question of getting permission does not arise. You can see the complexity and really, impossibility of this while traveling. In any case the photos in this section would have lost their spontaneous quality had this been the case.
This shot surprised even me. These friends were standing on the footpath as I passed them by in a taxi on the same trip as the above shots (most of them). It was highly serendipitous that I could frame them in this way so perfectly and the expressions were 'perfect'. There must be a good story behind this shot!
Taken from an open train door somewhere between Mount Abu and Delhi. Not such a 'Fairy Queen' scene. Looking more like a set from a play.