This week Sheldon and Amy will explore a whole new avenue in their relationship in a special Valentine's Day episode of the "The Big Bang Theory."

In the upcoming episode, titled "The Locomotion Manipulation," the couple will join Howard and Bernadette for a romantic dinner on a train bound for Napa Valley wine country.

"It is a very big, very sweet episode for Amy and Sheldon," added actress Mayim Bialik who plays Amy on the show, reports The Global Dispatch.

However, things don't always go smoothly for the pair. In a promo, Amy seems to be less than amused when Sheldon spends his time making weird noises with a fellow passenger rather than engaging with his girlfriend.

"Sheldon and Amy have a rough go because he's obsessed with the train's mechanics," teased Simon Helberg who plays Howard, adding "But there are some major steps forward taken, with a pretty beautiful ending."

"They have quite the romantic moment," says the show's executive producer Steven Molaro. "It surprises them both," added exec producer Bill Prady.

Sheldon and Amy's relationship won't be the only romance in the Valentine's Day-themed episode. "Lost" star Tania Raymonde will make a guest appearance as Raj's new love interest. Raymonde will debut play a cute and somewhat surly veterinarian who ends up taking care of Raj's sick dog Cinnamon.

"Leonard and Penny are watching Raj's dog Cinnamon, and they nearly poison her and kill her. So they all spend Valentine's Day in the vet's office trying to make sure Cinnamon is OK. And the vet might be a cute young lady," Molaro told TVLine.

Later on in the season, Bob Newhart is slated to make a third appearance on the popular sitcom o reprise his Emmy-winning role as Sheldon and Leonard's childhood Professor Proton, TVLine reports exclusively.

"The Big Bang Theory" S7E15, "The Locomotion Manipulation," will air on Thursday, Feb. 6 at 8 p.m. on CBS.

Watch a preview below.