The underground station I live next to in London is usually relatively quiet, however it is also the local station closest to the Wimbledon tennis courts, so for about 2 weeks a year it really comes to life, starting today. (Yay Wimbledon!)
I saw no JWs last year, but they seemed to have cottoned on to this opportunity and were out in force today. I think they must have started planning ahead.
Also as I was stood outside the station queueing to get in, the JW on the cart approached to offer me a magazine, I declined and told him he wasn't supposed to approach people, he ignored me and moved on to the next target. There was competition from the Jews for Jesus, who seemed to have no hesitation in handing out their tracts and I wondered if this had inspired the JWs.
Meanwhile the underground station for my work has always been a busy area for proselytising, it's right on Oxford Street, the main high street in London and one of the busiest stations. I've seen everything from evangelicals to muslims and even scientologists outside. JWs have caught up to this as well and are now outside every day! It's a major frustration that I have to see JWs nearly every day of the week now, I can't get away from these people! Bizarrely JWs are the only proselytising group that never approach anyone, but they're also there more frequently than anyone else. I'm told they're at all the busy London stations daily.
My point is that while you guys in smaller towns might not see the point in these carts, they're heavily targeting busy areas in major cities and increasing their presence. I don't know if this will be effective for them but I think I can see the logic behind this approach.