Here's when Spencer reveals his deal in Thomas' Place as a Prime.
Spencer: indeed, [drives up to Megatrain] Your army will soon be back.
Brian: Spencer, YOU'RE PART OF THIS TOO?!!!
Spencer: You are correct, I had made a deal with Megatrain.
Shining Armor: Spencer, you traitor!
J.J.: Spencer! Stop! Think about what you're doing!
Spencer: Hey, he looks like me, and I like it. I'm not just the Duke and Duchess of Boxford's privite engine, you know!
J.J.: You may only be the Duke and Duchess of Boxford's privite engine, but it's what's inside that counts! Besides, Megatrain will betray you!.
Spencer: Betray me? I highly doubt that.
J.J.: You can't do this!!
Spencer: [as Prince Hans] I think I already have. [he deploys a AN/M2 M1919 Browning machine gun]
Joe: He's got a gun!
[the rest of our heroes take out weapons]
Percy: Have you forgotten that we have weapons too?
Megatrain: Let them be, we'll deal with them later.
Spencer: Right, we have to bring your army back.
Megatrain: [picks up Spencer and deploys his flight pack] So long earthlings!! [takes off]
Princess Luna: We must be ready for him when he returns here.
Steamy: No! Ya can't let him get to the machine!
Spike: Why? Tirek already destroyed the Matrix!
Mucker: You don't understand!
Dazzlen: The Matrix is just merely a spare key, Megatrain's own claws are keys too!!
Twi's Friends: WHAT?!
Apple Bloom and Scootaloo: WHAT?!
Sweetie Belle: WHAT?!
Brian: ARE YOU SERIOUS?!
Zip: And if his Army returns, everyone is doomed!
Twilight: [her eyes well up with tears] Yeah, but Thomas is still gone. [starts crying]
Skyla: Don't cry auntie Twilight.
Twilight: I'm sorry Skyla. But he's gone. [countinues crying]
Brian: It'll be alright, Twi [pats her back]
Gordon: Well I'm going after Spencer!
J.J.: Me too!
Steamy: Let's go get that traitor!
[Some of the engines race off]
[Ernie peaks behind a tree]
Ernie: Okay, maybe conquering the world was a bad idea.
Spencer: This is fun! Don't you agree?
Spencer: [hears the other engines] Well, if it isn't Gordon.
Megatrain: You handle them while I do my things.
Gordon: Spencer, stop!
J.J.: You can't do this, you're our friend!
Spencer: Not anymore. [he fires his gun]
Ben: Look out!
[they avoid the shots]
Steamy: Spencer! Ya don't know Megatrain like we do!
Spencer: Really Because he seems like a nice guy. [fires again]
Uray: [takes out his plasma shotgun and blasts Spencer's gun] We're serious!
Spencer: [speeds away] Shoot, shoot, shoot, as fast as you can. You can't catch me I'm the evil train!
Gordon: You know, I think he should find out for himself.
Spencer: [to Megatrain] Now!
[Megatrain locks them in a cage]
J.J.: Spencer, please liten to me!
Megatrain: Shut up!