In the 6th book, Harry sees Dumbledore's memory, where he visits Tom in the orphanage and tells him he is a wizard. Dumbledore then departs from Tom:
"All the details are on the second piece of parchment in your envelope," said Dumbledore. "You will leave from King's Cross Station on the first of September. There is a train ticket in there too."
Dumbledore doesn't tell Tom how to get onto the platform, and we know it wasn't written in the letter, because also Harry didn't know how to get on.
It doesn't seem likely Dumbledore would forget to tell him such an important detail. Was it intentional? And how did Tom figure it out?