I could see that working as a last resort if the finances got really dire, but due to the nature of what surrounds the tracks Metra runs on, it'd be a hell of a sell for whatever advertising agency gets hired out to do it. The problem I see is that the cars get moved around from line to line, for mechanical reasons and for fuel reasons. A corporation could pay for a car (or train) of solid cars to be runing on the BNSF, which is almost consistantly surrounded by population, only to have the car get reassigned to the SWS, which obviously does not have the same population density surrounding it as the BNSF does. Advertisers wouldn't like this, and would start asking questions about why the Brand X's ads are seen by thousands every day on a busy line, but brand Y has its car on a line without many people.
The only line I could feasibly see this working on would be the MED, and I bet it would go over like a lead balloon among the ridership since that line already gets the short end of the stick in more ways than one (although it might keep the old Highliners from rusting any more...).
