And she'll find one of the barriers not solid, but more like an illusion or a hologram, and she goes straight through. On the other side:

A scarlet steam engine waits next to a platform packed with people. A sign overhead says Hogwarts Express, eleven o'clock. Behind her there's a wrought-iron archway where the barrier was, with the words Platform Nine and Three-Quarters on it.

Smoke from the engine drifts over the heads of the chattering crowd, while cats of every color wound here and there between their legs. Owls hoot to one another in a disgruntled sort of way over the babble and the scraping of heavy trunks.

The first few carriages are already packed with students, some hanging out of the window to talk to their families, some fighting over seats. 

Sarah will find a loo on the train and change into the robes she packed in her bag, she doesn't want to be wearing Aunt Petunia's hand-me-downs when making a first impression.

There are a few empty compartments but most have people in them already.

She'll look for a compartment with a couple people her age, preferably one that isn't too loud.

Here's one with two girls her age.

She'll walk in. "Hi, do you mind if I join you?"

The girls look up at her and the one on the left says, "Sure. I'm Millicent. Bulstrode. This is Pansy Parkinson. What's your name?"

"I'm Sarah Potter."

"The Sarah Potter?" Parkinson asks. "You're messing with us. No you're not, you even got the scar there. Blimey."

"It's nice to meet you."

"Nice to meet you, too! Where've you been?"

"I've been living in the muggle world."

Parkinson looks disgusted and Bulstrode horrified. "Why? Were you kidnapped?" the latter asks.

"I'm not quite sure, but I've been living with my mother's family."

"Your mum was a muggle?"

"No, she was a first generation witch."

"A muggleborn," says Parkinson, pursing her lips.

"Oh, is that what people call it?"

"Yes. ...I guess that still makes you half-blood..."

"That's an interesting term. I'm not sure I like the sound of it. It makes it sound like I'm less somehow."

They're both very carefully silent.

"It's alright, I understand." And she does, maybe she's lucky the Dursleys forced her to make up her mind for herself.

"Understand what?" asks Parkinson.

Ahhhh, what can she say that's not terribly insulting. "It's... words like that are sticky, everyone uses them."

"People with parents who are both magical are pureblood," Millicent says.