Forget security; I used SFO last week and just going from long term parking to where security is took 20 min, and the reverse (around midnight) took 30+ min. A train starting in downtown SF could as well be past San Jose at that point.
To be fair long-term parking at a train station would also be inconvenient (although at least in this case you'd not share facilities with all the international travelers), and honestly didn't even register as an option to me, so I was assuming local train/shuttle bus/cab/uber dropping off in a convenient central location.
I think the differentiating factor is that a rail station is more likely to be in the city center, where you can get to by other means (bus, BART, whatever), while going to SFO is much less convenient.
