I own a Carry Freedom City trailer ,
Carry Freedom Website | 3. The CITY Trailer it folds flat within it's own frame.
and has 2 rollers on the back edge to let it slip into the space between 2 back-to-back seats on trains.
the bag can be carried on your body. as can the Touring bag on the bike..
+ the trailer will also carry the folded up bike on top of itself ,
to say run quickly to a train car seat reservation, that may be at the far end of the platform.. trailer towed in hand.
I used mine recently to carry my guitar in its case , lashed down ..the neck end extending to the left over the tow-handle..
http://www.flickr.com/photos/88639757@N00/414714391
http://www.velovision.com/showStory.php?storynum=1226