This is not quite on topic but reminds me.... I was on a scuba dive trip in Tobago a few years ago.  After doing 3 dives for the morning we were heading back to shore.  I was by myself in the bunk room on a very small boat. It had been a relaxing morning of dives and the boat was gently plowing through the ocean.  I could hear other guest talking up on the deck.  The motion (and the thought of getting caught) made me wet.  So  of course I pleasured myself in a slow lazy manner.  After, I went up on deck and joined the others with a smile on my face..

Posted By: OnlyLiveTwice

Which provides the better rhythm for sex, in your opinion?  
   
 Assuming private quarters of course  
   
 

But I have to go with sailboat.

I've had sailboats of various sizes & descriptions...  I don't think I'd be able to sail & have sex.  I had a GF & we did it regularly in her docked cabin cruiser & also my runabout on the lake.  I don't think there was signifacant motion.  Also risked the campground dock late at night with a nice moon...  or 2.  Now that had some movement.  
Never tried a train.  Not sure what kind of car these days would have the necessary privacy.

I want to be ogled by my fellow passengers! Perhaps a threesome or foursome could arise..

But honestly, anything additionally moving sends me spinning into another dimension!  
But that happens on land too...lol  

Hey, we could turn this into a physics equation

sail boats don't have autopilot & you could get in a world of hurt with an unplanned jibe.  Perhaps a cockpit BJ or abchored fun as I did.   The run about had pretty high sides & we were able to lie on the deck & be out of view...  in some out of the way Cove.  (Missionary)  Busty Mrs Robinson
Small sailboats have the centerboard trunk in the way.  (Ought to be possible to sit facing side to side & connect in the middle)  

Now some lap play might be as possible as a canoe or ??   Had the most fun, neck deep on a quiet sandbar.  Loved the way the water made her nipples stand up!    

Once skipped classes to take my date & another couple out for a Spring sail in a good sized sloop.  Very romantic, a nice picnic lunch at anchor not conducive to further action.  (Besides having my relatives wave as my boat went by...  I waved back...  acted normal... )  Had I been further into that relationship we might have kicked the other couple out of the cabin for quicky...  Needed the extra hands to run the boat.  We had one heck of a sun / wind burn the next day.  Wish had that boat back now...  
   

-- Modified on 3/17/2015 9:52:56 PM

EOM

Once upon a time I owned a 27' Hunter sailboat, and was single at the time.  I sailed the Chesapeake Bay.  Lots of covers and creeks to anchor in.

Water made my lady friends very horny, so we did it in the boat and on the boat.  

There is noting better than getting blown, in the open, on a warm night, with a breeze so no bugs.

Time for a cold shower.

but love trains and would just at the chance to try it...