The 19-year-old was stunned when her compositions – “I Wanna” and “Don’t Want to Go Home” – were given the thumbs up by the film’s award- winning actor Timothy Spall.
Now millions will hear her music when The Last Bus opens in cinemas across the country on Friday.
It tells the story of an old man who travels from John O’ Groats to Land’s End carrying the ashes of his late wife in a suitcase, using only local buses.
The road movie was shot on location in Glasgow, Ayrshire, Helensburgh and Lanarkshire.
Caitlin said: “When I heard Timothy Spall liked my songs and they wanted to use them in the film I was in shock.
“It was like seeing my life flash before my eyes.”
Spall became a household name in the 1980s TV drama Auf Wiedersehen Pet, when he played electrician Barry Taylor opposite actors Tim Healy, Jimmy Nail and Kevin Whately.
In 2015, he won a best actor award at Cannes for his role in the acclaimed Mike Leigh biopic, Mr Turner.
Caitlin, of Viewpark, Uddingston, Lanarkshire, has revealed that her big break came after a chance meeting between her grandfather Jim Agnew and the film’s director Gillies MacKinnon.
Jim worked as a vehicle wrangler on the film, organising all transport, from vintage buses to luxury coaches.
Caitlin said: “It's amazing how things work out. My grandad and the director got chatting and when he told me he’d sent on a couple of my songs I honestly thought he was winding me up.”
But MacKinnon was so impressed with Caitlin's work that he decided others had to hear it too.
He said: “Often when you’re making a film somebody will come to you and say here’s a book I’ve written or whatever and give it to you.
“But in this case, Jim simply said I’ve got a granddaughter and she records songs and he gave me some background which was interesting.
“She was only 16 at the time and he asked, could I listen to them? I said okay, fine, send them on anyway.”
The next thing Caitlin knew, "Don’t Want to Go Home", about a road trip from London, and "I Wanna", about reaching out for comfort during a difficult time, were being considered for the final cut.
Gillies explained: “I listened and I just loved these songs. The emotion really embraced the soul of the film. I sent them down to our editor and she loved the songs and she put them into the film.
“And then I thought, well, maybe it’s just her and me who love these songs?
“But I began to learn that almost everybody who listened did.
“I brought Tim into the cutting room about half way through the cut – he’s an executive on the film –and he just loved the songs.
“Of course that influences me a lot because, as the person who walks in the shoes of the character, he’s really emotionally tuned in.
“So I began to think this is just one of these strange and wonderful things that happens, because Caitlin apparently sits in her bedroom and records with just
“And yet somehow there’s something within the songs that just catches a mood for the film.”
Caitlin, who busks in Glasgow city centre, started writing aged 15 while still at school. She is about to enrol on a music business course at college but is self taught, sings, plays ukulele, guitar and keyboards.
She said: “I just want to sing and write songs. My idols are people like Ed Sheeran and Taylor Swift as they are quite similar to my style.
“I use music as a way to express how I feel and just being able to get it out there and help other people that feel the same way, gives me a sense of hope.
“These songs just appear from nowhere, they are quite natural. They just kind of happen. Being able to have an audience gives me so much motivation.”
Her new album Through the Night is soon-to-be released on Spotify and Apple Music. For more information see @MusicAgnew on Twitter. The Last Bus opens on August 27 in cinemas nationwide.