I thought this, too, when I started a job in a city centre for the first time - great, I'll take the train, and get so much work done on my commute instead of driving!
No - it's impossible to work on a rattly, wobbly old train, standing up, or crushed into a seat with some guy's elbow digging into you. You can barely read a book or Kindle on there, let alone whip out a netbook and work. So it's just wasted time every day.
Pretty much. I've been experimenting with taking Amtrak from where I live in the east bay to the office in San Jose. The train is 2 hours each way, but it's like commuting on a cloud. I catch up on news or sleep in the morning, and finish my workday on the evening commute. it works because I can do 6 hours at the office and 2 productive hours on the train.
It does make for a very long day though. Between driving to my home station, taking the shuttle from the station to the office, the actual commute, etc, I'm up at 5:30am and get home at 6:45pm.
Connectivity is an issue too. Amtraks spotty wifi is fine for browsing -- queue up a bunch of tabs when the signal is good -- but it's terrible for anything else. Forget about ssh sessions or transferring large files or anything like that.
But, the thing is, the train trip into the city is 1 hour, and the drive is 30 minutes. And the train schedule is dictated by people who work 8 hours in the office; taking an off-schedule train is a much worse proposition. So even though I gain the time on the train (which is probably not as valuable as I think it is) I still lose out on the rest of my life for having 1 hour less per day to spend completely electively.
So, yeah, I'd like it if my car drove me into work so I was free to do other things. But I would still value a 30 minute autonomous drive much more highly than a 1 hour autonomous drive.