For some perspective, the cost of adding a lane of Interstate highway is estimated to be $1.6 million to $3.1 million per lane-mile. There is something like 45 miles of two lane highway on I-94 between Pewaukee and Cottage Grove per a quick Google Maps computation. Based on these estimates, the cost to widen I-94 from two to three lanes in each direction would be between $144 and $279 million dollars.
That is only one-fifth to one-third of the cost of creating Doyle's "high speed" rail line. A rail line that will have minuscule ridership, be expensive to ride, be extremely inconvenient for almost all riders, and take longer to get between Milwaukee and Madison for any conceivable purpose.