The actual answer to this question is that Muggle roads do cross the line of the Express just fine. The space isn't just hidden; it's folded, the edges touch. 

This is also why the surveyors don't notice; all their measurements in fact come out quite consistent, except of course for the size of the planet.

This is not the Ravenclaw table, though, and nobody in NEWT Transfig is paying attention to this conversation. (Well, to be specific, Andromeda Black is not currently paying attention to this conversation, as she is in a coat closet with her boyfriend. There are no other Slytherins in NEWT Transfiguration, because by the time they reach the age of sixteen they tend to have decided they're unwilling to have any more classes with McGonagall.) So the Slytherins all just sort of squint at her like she's said something obviously nuts that they're not sure how to contest.

" ... I think you probably want to ask a professor that one," hazards the history kid.

"I expect it's probably something to do with - oh, what was that lovely word in L'Engle's book - tesseracts, if the Muggles simply can't get into the 'missing' space and yet public works...continue to, well, work.  It's quite amazing that we weren't left in the dark for the ride up, then.  Or - what would that look like...Or maybe it's only actively pinched when there's a Muggle trying to look?  ...No, that would have Muggleborn children on the tracks unexpectedly...  Goodness knows I wouldn't know.  There's barely anything in the curriculum I have notes from about wards to begin with."

"Oh, yeah, wards is NEWT Runes, you need like thirty textbooks of prereqs or something," says the one who knew what 'perishable' means, sympathetically.

"She's gonna have read 'em all by this time next year," snorts Karina. She is only like 50% kidding.

"We are the house of ambition," she quips.  "Setting out to read the entire library, or at least the useful bits...  While it's traditionally a matter for Ravenclaws, it is nonetheless quite ambitious, by my reckoning.

"After all, knowledge is power - and who here wants to turn down power?"  Why, absolutely no-one!  Or they would be in Hufflepuff!  (Notwithstanding that the Hufflepuffs have the most weight to throw around by far, thanks to their sheer volume - but that's advanced political theory.)

"Is it even possible to read the whole library?" wonders the history kid. "Doesn't it technically have infinite books in it or something."

"I think no? Infinite shelves, finite books."

"... how does that even... you know what, I don't care, I dropped arithmancy for a reason."

Snort. "Still a rather large finite number, mind, but sounds like a respectable ambition to me."

"It's certainly not the end of my ambitions, but it is a decent start."

If this was a Bond movie, she would be petting a fluffy white cat right now.

She's petting a bird, instead!

Variously small Slytherins are not immune to the halo effect and the Baby Dark Wizard Vibes here are impeccable. She gets some approving nods, and then everyone's mostly busy with food and/or other unremarkable conversations until it is time for their next class, which in the case of the first-year Slytherins is Potions with the Hufflepuffs.

This is going to be an interesting class!

IF, and only if, nothing explodes.  She brought the required safety supplies.  She brought safety goggles.  She brought a fire blanket.  She brought an emergency squeezy bottle full of water for eye rinses, for all that she expects few problems to be solved by this.  She brought her best attempt at a mask - batting-stuffed, sewn from cloth and leather scraps in the mien of a plague doctor - to keep out volatile fumes.  She brought her best attempt at a labcoat, or, really, more of a lab apron - meant to quickly come off, in an emergency.  She is most assuredly not going to be wearing her hat when she enters the room.

"Good afternoon!" booms the large bundle of expensive tweed sitting at the front of the room. "Grab a seat, grab a seat, my dears, wherever you like, only try not to sit next to someone you'll be direly tempted to throw eel eyes at, hmm?" 

On the one hand, this is not as bad as it could be.  He is, at least, correctly concerned about avoiding mischief.

On the other hand: Why.

Is there anyone here who looks like they'd appreciate being the target of inter-House mingling?

Most of the Hufflepuffs are eyeing the Slytherin contingent with nervousness, disapproval, or both. 

One, a blond boy that Ophelia will recognize from the Sorting as Edgar Bones, is visibly trying to decide who his newest potential friend should be. 

She'll sit next to him!

And, before the Slytherins get too far apart for quiet words to fail to carry, murmur, "Ooh, hello, potential minion."

"Your funeral," Karina mutters dryly as she chivvies Annette into a back corner, glaring Avery and Rosier away from her preferred table. 

"Hello!" says Edgar, brightly, as the rest of their classmates pair off in-house. "How's the uh... evil Hogwarts experience?" 

He does not sound at all sarcastic, just genuinely interested, and has politely chosen to ask a question only because Ophelia will have at least a sentence or two of time to reply before everyone shuffles into their seats and Slughorn starts talking. 

She just Grins Mysteriously (and perhaps with a hint of fondness-amusement-joy) at him!

Well, no, she grins just long enough that he begins to think she won't answer the question in anything so pedestrian as words, before sideswiping his train of thought with a "Quite well, actually; I've already found a half-decent minion.  You may call me Ophelia when 'Miss Prince' is not suited - what shall I call you?"

(A lot of her approach, here, is just typical Slytherin posturing, designed for her deskmate to effectively ignore - but there's one tricksy little thing she's slipping by her deskmate and the Slytherins both - to wit, the reading of who is within the target group of minion.  To Edgar Bones, it cannot possibly be him, because she's befriending him and that's incompatible with minion-ifying.  To the Slytherins, it cannot possibly be them, because Ophelia hasn't done any minion-implying things to them (Yet.).)

This eleven-year-old definitely one hundred percent thought until just now that evil wizards having minions was just a made-up thing people put in, like, plays and adventure novels, for narrative purposes. Like sure, the Crabbe-and-Goyles exist, but 'minion' is just a mean name people call them, right?  

"....congratulations?" he hazards, fascinated. "I'm Ed, to my friends." And if he has his way that's everyone. "It's awful everybody calling me 'Mr. Bones' this week really, I keep expecting to turn around and see my big brother standing behind me even though he graduated four years ag -" 

Slughorn stands up, when everyone has mostly sat down, and Ed stops talking, perking up to listen attentively.

"Right then! I see you've all found your partners without too much fuss, well done. Welcome to your first Potions class! I look forward to guiding all your lovely bright young minds through, in my humble opinion, one of the most beautiful of wizarding arts. I am sure that many of you have heard all manner of nasty warnings about the dangers of potion-making, and while most of them are true, very few of them remain so within the confines of the Hogwarts potions classroom. Today we're just going to be getting familiar with the equipment; I will ask you turn to page four of your textbook, the cure for boils, and follow the recipe thereon only up to the end of the page." 

Her hand is raised.  She does not, actually, wait to be called upon before she speaks.

"Until which instruction, please?  Or rather, I imagine that you intend for us to avoid specific hazards with your aid, and I wish to know which ones."

She has a neatly-drafted annotated recipe sheet, printed upon centimeter-square graph-paper and then laminated.  It is not multiple pages long.  (She also, in the front pocket, has a glossary of terms and her best guesses at their meaning.)

Slughorn peers over his decorative pince-nez, visibly registering Ophelia for the first time as an individual human person and not as 1971 Slytherin First-Year #4, and contemplates her recipe sheet. "Gracious me, you are prepared. The last instruction on page 4 is number 36, which as you can see - I should hope you have kept the numbers the same in your, er, glass tablets? - instructs you to arrange your ingredients in preparation for adding them to the precisely heated cauldron." He glances around at the class, determines that they are mostly listening with curious interest and not whispering inattentively to each other, and continues. "The reason that I ask you to stop there is that when you begin adding the ingredients this will, even done perfectly, change the temperature of your cauldron, which I am going to measure as part of your grade for this class period. Next week we will repeat the procedure, for practice, and complete the recipe." 

"It's laminated graph paper, Professor."

She nods, produces a red dry-erase marker, and marks STOP, SIGNAL PROFESSOR in a precise hand over the relevant segment.

...That said, "it will change the temperature of your cauldron" is, at once, obvious, and yet incredibly underspecified.  Change how?  By what means?  At what rate?  To what intended result?