Hmmm, I didn't hear where he said that. I heard him say that linden trees are used along the tracks. I know my city bees visit a lot of linden trees because they are used for city landscaping a lot. That's a little different than "bringing in pollen on trains". City flows are often extended because of the diversity of plants in gardens and parks. From what I understand, plant diversity is a good thing. And often missing in the 'country'.
Don't knock city bees. Mine are doing quite well. I grew from 3 hives to 6 this year and gave away 2 and harvested 6 gallons of honey from the 2 established ones we pulled from. There are healthy bees in the city.