Advertising doesn't provide much revenue for the TTC. Check out the budget.
In 2018, that was $28.35m. Around the same as many of the add-ons that TTC has provided (2 hour transfer, under 12 free, etc).
https://stevemunro.ca/2017/11/14/ttc-plans-flatlined-service-and-fares-for-2018/
That's all...