[outoftheice]

To be fair, I don't think any railway will achieve 100% reliability. Even grade separated systems like Toronto's subway and the Tube in London don't achieve 100% reliability. For me the biggest concern for the Green Line isn't that a street running system will be less reliable than a grade separated system it is that the single bore tunnel now being proposed can put the entire line into failure when reliability does break down.

If you look at the current double bore tunnel design of the Green Line every underground station has a set of switch-over tracks designed as part of it. With this design if a train breaks down, there is a system fault or if there is an accident trains can use the switch-overs and a single track operation can be used for the small segment between the two stations where the issue happened. This minimizes the delay to the system and allows trains to continue running on the entire line.

With the singe bore tunnel design the tracks are stacked on top of each other and there is no ability for trains to switch over. In essence that means that if there is a failure anywhere in that underground portion that direction of travel is completely blocked. Passengers will have to disembark trains, get onto shuttle busses and then board again at a station further down. The longer the tunnel section (ie: extending it further up Centre St) the greater the odds are something will happen putting the line into complete failure mode in one direction.

I'm surprised this issue didn't get a single mention at the latest Green Line presentation at Council as I feel it increases far greater risks to the reliability of the Green Line than having a surface running train on Centre St