Leonard, Sheldon, and Howard are nearly finished with their device, but Sheldon has a way that they might be able to make it even smaller.

His theories are slowing the guys down, so Leonard pulls out the big guns. He has a way to distract Sheldon that he's been holding on to for a review years now -- a special trip involving trains. 

Sheldon is so excited that he forgets about everything else.

Meanwhile, Bernadette is overwhelmed with the baby, and Penny suggests they have a girls' night out.

That means leaving baby Halley with Stuart and Raj, who unfortunately keep calling Bernadette and make her question whether or not she should just go home.

The colonel visits Howard and Leonard and notices Sheldon's equations. When he realizes they are for a smaller nagivational system, he insists that the guys build that version instead. 

Sheldon refuses to help, though, because he's now focused on his trains. He doesn't even fall for it when they guys try to write a false equation thinking Sheldon won't be able to help himself but to fix it.

Penny starts talking about her relationship with Leonard and her worries that they aren't moving forward, and all the women talk about growing older.