Howard and Leonard try to make their invention even smaller without Sheldon who's busy trying to assemble a working train set. Raj and Stuart look after Halley while the girls take Bernadette out.
The guys have finished working on their gyroscope making it the miniature size that the government wanted. Sheldon says that he can make it smaller. The guys are glad to be finished and send Sheldon away. Col. Williams comes in, sees Sheldon's work and wants it the smaller size. Despite Leonard and Howard pointing out that they've finished the gyroscope to the specifications and that they can't make the smaller size, Colonel Williams demands that they make it anyway.
Penny's Posse is meeting at Bernadette's house and decide to go out dancing. Amy wants to go see some Renaissance choir. They leave with Raj/Stuart watching Halley.
Amy gets home and Sheldon is excited about a train they could have running all over the apartment.
In apartment 4A, Leonard is trying to duplicate Sheldon's work to make the gyroscope even smaller. Sheldon is in 4B assembling his train track in a welder's outfit.
The girls are on their way dancing when the babysitter's call to say that her milk she left smells funny. Bernie is okay for them to use the spare milk. She also doesn't want Stuart to use it in his coffee.
As the girls arrive to the bar, they discover that it was turned into a bookstore. Just then, Raj phones Bernadette when Halley starts crying again. Stuart points out that it was from Raj taking a selfie as Halley blew a spit bubble while sleeping. Halley eventually falls back to sleep. At that moment, Penny changes her mind about staying out for night, even though Amy and Bernadette still want to. Penny then has a breakdown because ever since Bernadette had her baby, she believes that her life hasn't gone anywhere since marrying Leonard. However, both Amy and Bernadette point out that Penny has a lot to be thankful for: she has a successful career, a nice apartment, and is fortunate enough to have Leonard, a man who never hesitated to marry her twice, in her life. Amy, on the other hand, starts feeling miserable as she doesn't have what Penny has: her salary and her apartment aren't as big, and she hasn't even been engaged yet (Sheldon was ready to propose to her twice, but isn't following up on it anymore). This causes Amy to cry, which causes Penny to cry as well, much to Bernadette's annoyance.
Leonard is finishing his calculations and wants to trick Sheldon into looking them over. Later Amy is dreaming about Sheldon behind a huge locomotive all sweaty as they head out to see some train.
Thanks to Fandom Fussion 
- Guest starring:
- Teleplay: Steven Molaro, Maria Ferrari & Tara Hernandez
- Story: Chuck Lorre, Steve Holland & Jim Reynolds
- Title Reference: To be entered.
- Taping date: January 17, 2017
- This episode was watched by xx.xx million people with a rating of x.x (adults 18-49).
- Total viewers including DVR users xx.xx million.
- The Big Bang Theory was ranked #x for the week ending xx February 2017.
- This episode aired in Canada on February 2017.
To be entered.
- Leonard, Sheldon, and Howard finally finish the gyroscope for the military, but are forced to make a smaller gyroscope when Colonel Williams sees Sheldon's plans for one.
- Sheldon begins to take an interest in engineering (though he claims that it's not the same as Howard's engineering.)
- Bernadette has her first girls' night since giving birth to Halley.
- Sheldon once again mocks Howard for being an engineer, despite signing a contract where he promised to no longer do so.
- The song played during Amy's fantasy about Sheldon is "Rock You Like a Hurricane" by The Scorpions.
- Sheldon still hasn't done anything regarding his engagement ring since Amy found out about it, prompting her to wonder if he really wants to marry her anymore.
- Amy gets turned on when fantasizing over Sheldon controlling a steam locomotive.
- Sheldon (to Amy in her fantasy): May I borrow your water?
- Sheldon: The light's green.
- Amy: Sorry.
- Sheldon: GPS says we'll be at the railway in three hours.
- Amy: I can get us there in two. (steps on gas)