I just wanted to throw out some numbers and facts out there, while i know im new to this site iv been a long reader, and recent wdw CM, so if you want to believe or not believe my facts is up to you, but i did hear them internally from some higher up people.
When wdw last reviewed the monorail expansion, the cost came to $130million per mile, while each new train would be $40million and can carry about 300 guests. Also the switch track where the new tractor hold is on the epcot line is not capable of switching a full train, as was the original intent. In terms of buses, they received an order of 50 new buses last june, and another 23 this past january. Each new bus costs about $230,000 (+/-) and can each hold 72 guests. The plan is to have about 400 buses and 2,000 drivers by this summer, where as of a few months ago it was about 310 buses and about $1,500 drivers. (pre januray delivery of busses).
So my opinion: its obviously not impossible and they do review it every once in a while, but its not practical at this point, the property just hasnt seemed to been designed for a mass rail as previously mentioned. Just my own preference, i think a light rail system could be a better fit, but thats just my own preference, nothing official.