The thing with transportation engineering is that it's but one of many specialties in the civil engineering program. If you enroll in the undergrad program, there will be at most 5 courses in transportation, with the rest of the 40 comprising of anything from structural to calculus to municipal.
Then once you finally graduate, don't expect to be working on cool transit projects all the time. For one thing, there has to be a demand in the first place. No new subway lines means no chance to plan a new system map. The bulk of the work related to transportation is done by consulting firms, not actual cities or transit systems. You'll probably end up working for one such consulting firm, which means that most of the time you'll be determining what modifications have to be done to the road system to accomodate new traffic from random developments.
To summarize, it's true that getting a civil engineering degree is by far the best way to get yourself into the transportation field. However you truly have to have a passion for every aspect of the field - not just have an interest in transit or smart growth - in order to be happy on the job.