does public transport advertising create demand or fulfil it?

One element of the job of a marketing manager is to sell the benefits of public transport through a variety of select mediums.

Here's a couple of ways you could approach this challenge.
  1. Use your work to persuade people to travel in the first place and hope they'll use your services as a result - creating demand for your product and hoping for the best.
  2. Put your bus or train company into the mix when you know a person has already made the decision to travel - trying to take advantage of a demand you know already exists.
Of course, you can do one of the above, both of the above (get you with your massive budget!) or none of the above but you probably wouldn't be great at your job if you chose the latter.

Your choice may be determined by the mode you represent, how your brand is perceived and whether your services are in competition with another operator.

Bus and train companies tend to look at this conundrum in slightly different ways.

My observations would suggest most train advertising aims to create a demand for travel by highlighting where you can go whereas bus advertising concentrates on a decision already made ('get to the gig by bus' etc). Maybe something to do with the distance each can cover?

There's no go-to answer, so whatever your approach, make sure your advertising informs, entertains and brings a smile to the faces of your potential customers!