You won't probably won't see any platform staff jobs openly advertised because TFL know that they'll get more than enough applications from people on their mailing list, who'll reply to the emails TFL send out when jobs become available.
Carry out a search (as you seem to have done before) here:
https://careers.tfl.g...tfl01SSL.asp?newms=se
(Select 'Frontline staff' as your option).
When you get the inevitable message telling you that there are no vacancies, enter your email address and click 'Save details'.
TFL might get at least 100 enquiries or applications for every vacancy which comes up, solely from responses to their emails. Openly advertising vacancies could possibly push the number of applicants into four figures (making selection a total nightmare), so it's in their interests to only inform people on their mailing list about jobs.
Chris
PS: I used to run a (mainline) railway station. I hope you enjoy dealing with abusive & threatening drunks and druggies, while simultaneously trying to give directions to non-English speaking tourists and helping someone else locate their lost property!