I am not an expert, but here are my thoughts... Currently the largest train carries 36 passengers, right? Figure even if all passengers are 250lbs, (a football team, it could happen) that makes for 9000 lbs of people. Add to that the weight of the train (???) Multiply that by the G-forces created on the average high stress points of a good coaster. (4.5) That is 40,500 lbs of people (the football team) plus the weight of the train. That is alot of weight. I have a feeling that too many more riders would, like you originally thought, would cause an over-stress on the average design of a coaster.
I have a feeling that there is a "golden ratio" of weight-of-train compared to the design-of-track that someone, somewhere has already figured out, and that is why you do not see more than 8 rows of 4 on modern coasters.