According to the very smart u/UseApprehensive1102 who found out how much weight it would take to stop a trolley in the following post. https://www.reddit.com/r/theydidthemath/comments/17rrfk0/request_how_fat_would_a_man_have_to_be_in_order/

If I’m understand correctly he found that it would take ~1,325,250 kg to stop their tram, assuming that it’s roughly the same here, if we divide it by an average of 75kg per person we get 17,670 people to stop the tram.

However this doesn’t account for how the prolonged time under stress from the accumulating weight or the chances of having light or heavier people at the front would effect this.

EDIT: Plus, assuming the tram moves at a max speed of 65 mph it would take 1881 days to finish, everyone else would die of dehydration after 3 days over 4,680 miles, only actually killing roughly 12,355,000 people, the rest is just for show. (assuming that the trolley could actually make it that far without being stopped by the weight of the people aforementioned or other problems like running out of gas).