One important distinction is that is not exactly a train to the airport, but a vehicle of mass transportation between Sao Paulo and the city of Guarulhos, where the airport happens to be situatted. It envolves several transfers on the subway sistem (2 or3, depending where you are located) till you reach the train properly. The stop in Guarulhos is not walking distance to the airport, so involves a bus, sounds awful but actually isn't, since is a shuttle, is already there waiting for the train, gets filled quickly and leaves, for a less than 10 minutes ride. Costs: nothing if you are already on the subway system. Duration: less than an hour, including waiting for the train, that leaves each 20 minutes. Comfort: looks like a regular subway train. Wasn't crowded when I rode early in the morning, let's see if going stay this way. I had a regular size luggage, a carry-on and a back pack, it went on fine, if I had 2 regular size bags I would take an Uber. To short the story: is somewhat cumbersome, if you have discretionary income and plenty of time take an Uber, if you are in a budget take the train, if time is a concern also take the train, you can be sure to arrive in an hour, where the traffic god only knows, might take an hour, might take 3. If you are a first time visitor or is not familiar with the subway system skip the train.

If is raining the weather tends to cripple road traffic so it may be more feasible in those conditions to take the Metro but give more time for the shuttle portion. A shuttle in rain to a nearby airport hotel from my arrival terminal mid-evening took over an hour door-to-door.

The road used by the shuttle is different from the road that reaches the airport from the city. Might have been built especially for the train (not 100% sure about it), I did not see much else around, so probably heavy traffic will not be an issue and the entire travel fits in the one hour time frame. Let's wait for other members posts to be sure about that.