Sunrail covers a fairly wide area, and often times these trains aren't about making it no driving, but much less driving. It may make sense for a local to drive to a Sunrail station, transfer to Virgin, to avoid driving and paying for parking and dealing with the I-4/Disney traffic. We'll have to see the pricing for the Meadow Wood to Disney ticket.
Right now in South Florida, they have a BOGO offer and kids are free... and the round trip price from Miami to Ft Lauderdale is a grand total of $22 right now for 2 adults + kids. Cheaper than Disney parking, not even including the cost of driving.