Will he be an ally for Harry? Possibly. He likes to be associated with successful people and ride their robetails, so I could see him being helpful to "the chosen one." He might even buy Harry's theory that Draco Malfoy has the Dark Mark and is a future Death Eater. Of course, with a mom and pop loyal to the Dark Lord it only makes sense.
Malfoy sounded convinced that Voldemort will rise to supreme power, and Malfoy couldn't bear being on the losing side.
Which brings us to Harry's foray into the Slytherin compartment. He is desperate to convict Malfoy and is willing to risk much (including a physical beating) to get the evidence that his rival is indeed loyal to Voldemort. He said as much even though he didn't say too much.
Because I chastised Hermione for her lack of spy craft in Knockturn Alley I should do the same with Harry for getting caught. Hastily devised plans often fail.
School is about to start, and it looks like from the chapter names that I'm only two away from learning why this book is titled "The Half-Blood Prince."
I've ridden the Hogwarts Express at Universal in Orlando, but I've always wanted to ride a real train to real places and see real countryside. Add that to the bucket list.