I was thinking of just fronting up to the ticket window for tickets for Dad and I from Shenzhenbei to HK W Kowloon, but in the end I booked a specific train on Trip.com and copped the booking fee, which wasn't as stark a proportion as
@Flashback 's ticket just upthread.
Especially when Dad opted for first class, despite it being approx double cost and for a 18 min ride.
The irony of this ride is that the border controls take far, far longer than the journey itself (which is pretty much one long tunnel ride).