TTC riders relieved at this year’s fare freeze would be advised to savour that break while they can, with the transit system’s budget challenges expected to grow incrementally in the coming five years.

The extra $91 million the city is poised to approve on the TTC’s $1.3 billion operating budget this year could grow to more like $400 million extra by 2014.

This year’s increase will come out of a $238 million provincial subsidy that isn’t likely to be forthcoming next year, even though the TTC will need a similar increase in 2010 to maintain last year’s service boosts and improve bus frequency to a minimum of 20 minutes.

There are also incremental increases on the books for hiring more staff, such as route supervisors and cleaners, and ongoing salary commitments to unionized workers.

All that makes fare increases inevitable, according to some city councillors

But “timing is important,” says TTC chair Adam Giambrone. “Right now, during difficult times for everyone, the city has recognized that freezing fares temporarily is a way of helping stabilize things and literally keeping people and the economy ­moving.”

The last fare increase, in November 2007, raised the price of tickets and tokens by 15 cents and hiked Metropass costs to $109 from $99.75. It added $35 million to the TTC’s revenues. But there was no increase in the $2.75 cash fare.

An across-the-board increase of 25 cents on the cash fare, which would take it to $3 — the same as on Mississauga Transit — would translate to a monthly Metropass cost of $121 and raise about $47 million.