As I completely agree with her recs, and I haven't posted a themed rec list in a while, it seemed like as good an excuse as any to post one now :)
Fics where the boys have sex on the Hogwarts Express:
A Funny Thing Happened On The Train To School… by treacle_tartlet (NC-17 | 1K) 25-Dec-2009
Summary: [no summary]
Yum! Draco just up and deciding to seduce Harry on the train. I liked that there was a sense of vulnerability and also determination. Short but great.
Second Time's the Charm by sophie_french (NC-17 | 2.5K) 10-Jul-2014
Summary: Back in sixth year, Harry had spied on Draco on their way to Hogwarts, hidden under his Invisibility Cloak. Back in sixth year, Harry had ended up with a broken nose. Now two years down the road, in a similar situation, he might very well end up with something else altogether.
Oh, this is surprisingly hot! Harry gets an eyeful and Draco doesn't mind as much as you might think he would :)
Polyjuice Pastime by dysonrules (NC-17 | 23.5K) 8-Aug-2007
Summary: The Polyjuice story that began with a bored Harry and now just won’t quit.
Super hot, and I love the dominating side of Draco (the marking/possessive stuff is very hot), even though Harry is an aggressive bottom. This is a WIP that seems to have been abandoned, but there's plenty of good stuff here anyway.
On the Hogwarts Express by Renee Spelt Strange (NC-17 | 9.3K) 7-Apr-2012
Summary: In which there is half-clothed hate-sex against a carriage window on the Hogwarts Express heading back to London. Except that...it's not really hate that Draco's feeling.
This is a good character study of Draco and a cool scene. Well written and interesting.
An Old Habit by fireflavored (NC-17 | 9K) 30-Apr-2009
Summary: The boys have changed a lot over the summer after the war, but Harry hasn’t got over the urge to spy on Malfoy.
Nice beginning, but mostly PWP. Featuring a very experienced Harry and a Draco who's spent the summer working with dragons :)