If the 401 rapid transit line was viewed in the same regard as the 401 itself, it might work. Hardly anyone starts their trip exactly at an on ramp and ends their trip exactly at an off ramp. Instead, their trip starts some distance from the on ramp, ends some distance from the off ramp, and the highway itself is simply a high speed connector.
In order for 401 transit to work, there would have to be an extensive, convenient, and complete network of intersecting transit lines. There are many people who might live near Jane and work near Don Mills who could use the 401 transit line to connect the two. Trains on the 401 itself would have to be frequent, high speed, and stop only at intersecting subway and LRT lines.
Toronto does not have anything close to an extensive network of high capacity north-south transit lines. Therefore, Toronto is in no way ready for a 401 transit line. Let's check back in a decade and see where things stand.
Note: I would hazard a guess that on the inner 401 between Pickering and Mississauga, most commuter trips average 15-20 km on the 401 itself - trips along the lines of Pickering to the DVP, 400 to the airport, or Yonge to Scarborough. Transit trips would mirror this typical distance, so don't expect throngs of people to travel 75 km from Pickering to Mississauga. This is why the ability to connect to north-south transit lines is of utmost importance. The 401 is primarily used as a connector - not to service points actually along the 401. 401 transit would be the same.