Is there a canon answer that explains how the Hogwarts Express was put into service?
Per Pottemore's article on the Hogwarts Express, it was originally a Muggle train (Exact origin was never proven). It has been magically altered, however, so it's absolutely canonically verifiably a Muggle train with enchantments.
You'll have to sign up for Pottermore to read the article from J.K.R., as it's copyright protected, and I'm not sure how much I could legally copy here.
As David Stratton mentioned, J K Rowling's article on Pottermore regarding the Hogwarts Express mentions the origins of the train:
"A daring and controversial solution to the thorny problem was finally suggested by Minister for Magic Ottaline Gambol, who was much intrigued by Muggle inventions and saw the potential in trains. Where exactly the Hogwarts Express came from has never been conclusively proven"
Later, it confirms that enchantments were placed on it:
"The Hogwarts Express underwent several magical modifications before the Ministry approved it for school use."
I think the Hogwarts Express is a regular train with enchantments, because it has all features of a normal train and it uses the normal railway-system.
A Muggle train with modifications
According to Pottermore, the train was stolen from a Muggle rail company in Crewe:
Where exactly the Hogwarts Express came from has never been conclusively proven, although it is a fact that there are secret records at the Ministry of Magic detailing a mass operation involving one hundred and sixty-seven Memory Charms and the largest ever mass Concealment Charm performed in Britain. The morning after these alleged crimes, a gleaming scarlet steam engine and carriages astounded the villagers of Hogsmeade (who had also not realised they had a railway station), while several bemused Muggle railway workers down in Crewe spent the rest of the year grappling with the uncomfortable feeling that they had mislaid something important.
That said, the train is certainly far more than a mundane Muggle conveyance, as we see in Harry Potter and the Cursed Child.
The TROLLEY WITCH’s hands transfigure into very sharp spikes. She smiles.
TROLLEY WITCH: So please retake your seats for the remainder of the journey.ALBUS: You were right, Scorpius. This train is magical.
There doesn't seem to be too many large scale enchantments that exist merely to simulate the functions of something real. Magic users didn't enchant themselves a whole house, or a bus, or a motorcycle, or a sword. Material stuff is built, then enchanted to be enhanced, in Harry Potter universe.