Last night (this morning?) I did the Boston Midnight Marathon. A couple of guys have been organizing this for a few years now, and participation trends seem to mirror that of the keg ride. Last year it was 200 something. This year I heard numbers ranging from 450 to 700. Hopefully someone comes out with an official count.
The marathon is a one way thing, rather than a loop, and as per the OP, it starts way out in the burbs.. 26.2 miles out in the burbs. The ride was supposed to start at midnight. Some people carpooled out, others rode out, but most took the commuter train (photos courtesy of the corresponding twitter handles):

dwillen - At South Station, Boston, MA - around 10pm

smoorearch - loading up

dwillen - mid loading

smoorearch - almost done!

jmevans00 - loaded up...

BostonTweet - all the train cars were unique as far as the seating arrangment went, so stacking bikes was a bit different on each one.

leetoma - a different car...

JackSullivan19 - another...

skurpie - my favorite
The MBTA was really, really awesome. The ride organizers notified them a couple days in advance, so instead of their usual Sunday night, 11 pm train, they brought out a train with extra cars. We started boarding an hour before departure and filled all the cars in only 15 minutes. They added more, and more, and more cars until everyone was on. We had over 13 train cars full of bikes and cyclists. This worked great except that all the other train platforms are only long enough for something like 4 train cars. They had to let everyone out in shifts. Let the first cars out, move the train up, let the next group out, repeat.
After we all got out and gathered in a massive group, it started pouring rain. It was about a 3 mile ride to the start line, then you had 26.2 miles till the finish. The course was marked, the outhouses were out, banners everywhere, but the roadway was technically open to traffic. We had a bunch of spontaneous police escorts at the start of the ride (I guess they figured they couldn't hassle 700 cyclists, so may as well support them till they gtfo). The rest of the course, I was passed by only one car, and saw maybe 4 cars coming from the other direction before we got into Boston proper. Not a lot of traffic that time of the night on a Sunday. The finish line was blocked off from traffic already, so we had a little place for people to gather as they finished. All in all it was tons of fun to get out on a nice evening with hundreds of other riders.