The "Red Train" occurred last year for everyone that rode the train from Chicago to Evanston for the NW game. I only saw one NW fan on the entire train when we got off.
If someone is really looking for a business venture, then something similar to Megabus is much easier to do. Pick people up in parking lots around Omaha, drive the bus to the game. Repeat for the return trip. Multiple routes between Lincoln and Omaha provide easy rerouting if necessary. No negotiations with freight lines. Pick-up and drop-off in places MUCH better than the Omaha Amtrak station. Easy to have multiple pick-up and drop-off times as well as locations. Can easily change the capacity based on how many tickets bought in advance.
I think the major problem with mass transit is cost and timing. If more than 1 person is in a car (which I'm willing to bet is the vast majority of people going to games), then gas cost per person is going to be less than almost any mass transit option. Timing is important as someone mentioned because you have to get to the pick-up location early in both directions, which adds anywhere from 20 minutes to an hour to the total travel time. Also, timing is important when people aren't on a strict schedule. A lot of people head down to Lincoln and then stay after the game tailgating, dining, drinking, etc. and don't leave until those events are over. Having to "catch the train" can be inconvenient.