The tracks appear to be visible out in the open; Ron and Harry could easily follow them in COS.
The Hogwarts Express tracks are probably hidden from Muggles.
It seems most likely that the tracks for the Hogwarts Express are hidden from Muggles. It’d almost certainly have to be hidden from Muggles in some way, since the Hogwarts Express passes by populated areas on the way from King’s Cross to Hogwarts, where Muggles could possibly see it if it wasn’t hidden from Muggle sight or notice somehow. When Harry and Ron follow the Hogwarts Express in the flying car, they (and therefore the Hogwarts Express) pass though London, villages, and a city in addition to countryside that wouldn’t be particularly populated. The city was almost certainly a Muggle city, since it had a lot of cars, which are typically used by Muggles.
“They made regular checks on the train as they flew further and further north, each dip beneath the clouds showing them a different view. London was soon far behind them, replaced by neat green fields which gave way in turn to wide, purplish moors, villages with tiny toy churches and a great city alive with cars like multi-coloured ants.”
- Harry Potter and the Chamber of Secrets, Chapter 5 (The Whomping Willow)
There’s further evidence that something was done to hide the Hogwarts Express from Muggles as well. The JKR writing on Pottermore about the Hogwarts Express says there are secret records in the Ministry about the largest ever mass Concealment Charm being performed in Britain when the Hogwarts Express was put into place by Muggles who were later made to forget they built it.
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.
- The Hogwarts Express (Pottermore)
Most likely, the Concealment Charm was to hide the Hogwarts Express and the tracks it traveled on from Muggles when it had been put into place. As it was put into place by Muggles, it couldn’t have been concealed so the Muggles building it wouldn’t know about it - they would have simply been Obliviated after their job was done. Therefore, the most likely conclusion for what the large Concealment Charm was for was to hide the train and its tracks from Muggles.