The shower curtain with the periodic table on it that was always hanging in the bathroom of apartment 4A (Sheldon and Leonard's former apartment) is now seen hanging in apartment 4B (Penny's old room) for the first time. Sheldon probably didn't bring it along when he moved his possessions in The Property Division Collision (2016), because Penny's old curtain could still be briefly seen while Amy was trying to seduce him in her wizard robes in The Birthday Synchronicity (2016). He must have retrieved it somewhere between episode 11 and 15.
The Nevada Northern Railway is an actual, operating railroad museum in Ely, Nevada (which really is 4 hours from any major airport!), and does offer rides, including allowing you to play engineer and operate the throttle on the moving train.
Howard's paternity leave has ended, and he returns to work in this episode.
This episode was taped on January 17, 2017.