The original version of "Only You" is a short duet reprise of "Only He", Pearl's solo.
Pearl, injured and disconnected at speed in the final race, has had time alone to reflect on her actions, and has realised that Rusty is indeed the engine of her dreams. However she has treated him so poorly, she doesn't believe she deserves his affections. But unknown to Pearl, Rusty left the celebrations at the finish line to find her. He turns up and they have a short conversation before Rusty confesses his love to Pearl, and she realises he found The Starlight, and won the race, and all is well.
The original structure of the 11th hour love song was that Pearl sang a show-stopping solo "Only He", and this reprise/coda scene advanced the plot as Rusty and Pearl reunite. The second iteration of the show, the 1987 Broadway production, simplified the vocal demands on Pearl, and streamlined this scene by combining Pearl's solo and Rusty's entrance, conversation, and duet into a single song, also called "Only You".
|Rusty:||Pearl, I had to find you, why you looking blue?|
|I'm sorry you never found your Dream Train|
|Pearl:||Rusty! I have found him, but you were looking too?|
|For the Starlight Express, did you have much success?|
|Rusty:||Yes I found him ok, now I'm brave enough to say|
|Only You (You are the Starlight!)|
|have the power to move me|
|Pearl:||You won the race?|
|Both:||And together we'll make the whole world move in sympathy|
|you and me, we'll be sublime|
|Both:||has the power to move me|
|Take me, make me, hold me, mold me and improve me|
|Only You have the power to move me!|