Yesterday as temperatures dipped down past -20 C, the TTC had to take some 35 streetcars out of service.

The problems meant many passengers were left standing in bone-chilling temperatures waiting for streetcars. A similar problem occurred last week when 50 streetcars were taken out of service.

TTC CEO Andy Byford appeared on CBC Radio's Metro Morning on Wednesday to explain why some TTC streetcars can't cope with the cold.

Byford said the decades old vehicles' "critical weakness" is the pneumatic air lines used to operate the brakes and doors. TTC crews purge the lines at night but during the day, moisture in the lines can freeze and prevent the doors from operating.  

"The main problem is the sheer age of these vehicles and the extreme temperatures," Byford told Metro Morning host Matt Galloway.

Disabled cars block the way for other streetcars, creating cascading delays.

So what can be done?

Byford said the main solution is to continue the program to replace the cars with newer vehicles. The new streetcars have hydraulic braking and door systems.  

New streetcars are being phased in, starting this year. In the meantime, TTC workers will continue to, in Byford's words, "work miracles" to keep the ageing fleet going until they can be replaced.

The cold weather also causes rail switches to freeze and ice to build up on the overhead power lines. TTC "response teams" were dealing with those problems frequently on Tuesday.

In the midst of yesterday's streetcar problems Mayor Rob Ford — an ardent advocate of subways over surface rail — tweeted: "With persistent delays on our streetcar system due to the weather, days like today remind us why we must invest in underground transit."

Byford said a mix of surface and underground rail is optimal.

"We are throwing everything at getting these ancient vehicles out on the road today."