The second time that Leonard and Penny suggest making out on Sheldon's spot on the couch but don't end up doing it. First time was in The Pirate Solution (2009), when Howard interrupted them.
Penny is slightly disappointed that the sex with Leonard took only 9 minutes. Leonard's inability to last out longer is joked about for the first time since season 4. Previously, he was told by Leslie Winkle in The Bat Jar Conjecture (2008) that "it's not going to work if you rush me, you have to let me get there"; in The White Asparagus Triangulation (2008), Raj called him 'Speed of Light Leonard'; in The Precious Fragmentation (2010), when Sheldon asked Leonard "what do the two of you talk about after the coitus?", Howard taunted Leonard by replying "My guess is, 'Hey, four minutes! New record!'"; and in The 21-Second Excitation (2010), when Leonard cannot convince Penny why Raiders of the Lost Ark (1981) with an additional 21 seconds of footage is cool, Raj claims that 21 seconds is also the reason why they are no longer together.
Title reference: Sheldon feels that Amy is tricking him into romance by taking him on a train trip.
The #6131 Q2 class locomotive Eric describes was a real locomotive built in 1944 in the 4-4-6-4 arrangement. But it was built by Pennsylvania Railroad, not Pacific Railroad as Eric states.
This episode was taped on January 14, 2014.