Multi-city flight with high-speed train

James, you are definitely allowed to proceed with your plan because it is eligible for 144 hour visa free transit that is valid for the entire Guangdong province. Please, just have in mind that your segment from Toronto to Guangzhou is not allowed to have any additional landing within mainland China of any kind. Also, you should have in mind that this is not 144 hour visa, but 144 hour visa free transit or in other words, there is no visa of any kind.

You are allowed to travel within entire Guangdong province as you like, including taking a high speed train from Guangzhou to Shenzhen.

Here the only detail is well worth emphasizing. When doing your itinerary about your visit to Guangzhou and entire Guangdong province, there is one detail you have to have in mind. The ticket that will take you from Guangdong province to Hong Kong, being it Guangzhou to Hong Kong or Shenzhen to Hong Kong, you have to book in advance, even before your departure from Canada. This is the proof that you will be really in a transit. When you book your train ticket you will print out a confirmation for the ticket and have that printout at hand, ready to be presented to relevant authorities in both Toronto and in Guangzhou. Also, keep in mind, you will really have to take exactly that train you have bought your ticket for.

Tickets for high speed trains have dates on it, although it is now mostly done by scanning your passport as a mean of entry into the train. A confirmation that I was talking about is a detail that you should print and have with you at hand. To conclude, you are good to go and I wish you a pleasant journey.