The eldest son of Harry Potter and Ginny Weasley takes the Hogwarts Express from King’s Cross at 11 am. After traveling all day, he is transported by boat across the Lake and ushered into the Great Hall. There he is sorted into Gryffindor House, to the dismay of Teddy Lupin, who is Head Boy and a Hufflepuff (JKR:Tw).
J.K. Rowling Tweeted in September of 2015 about James Sirius riding the Hogwarts Express for the first time, and being sorted into Gryffindor:
I'm in Edinburgh, so could somebody at King's Cross wish James S Potter good luck for me? He's starting at Hogwarts today. #BackToHogwarts
— J.K. Rowling (@jk_rowling) September 1, 2015
This notice actually appeared on a station platform monitor (although, sadly, not at King's Cross) on that day: