According to Wikipedia, about 3.4 million people pass through Shinjuku Station everyday. That is only one stop on the very busy Yamanote Line, which also has Shibuya Station (2.4 million passengers per day), Tokyo Station, and several other busy stations. Over 12 million people live in the 23 wards of Tokyo, and over 30 million people live in the greater Tokyo area, the majority of whom commute into and out of Tokyo by train.
By installing these in the ticket gates, there is the potential to generate a lot of power from relatively few generators, and it doesn't just have to be at train stations. Anywhere a large number of people is funneled through a small space will do nicely: ticket queues and gates at stadiums, amusement parks, airports, etc.
Of course, one thing that I am curious about is the durability of these generators. For example, JR Shibuya Station has somewhere in the neigborhood of 30 ticket gates spread over 4 exits. 2.4 million people walking through 30 gates everyday probably means a lot of wear and tear. How long do these generators last? How much to they cost to maintain?
Still, it seems like a brilliant way to generate an alternative source of electricity and perhaps take a large part of the load off of main power.