ScotRail Alliance managing director Phil Verster said engineers would be deployed at stations along diversion routes so they could deal with any problems immediately.
This is because services such as the main Edinburgh-Glasgow route will be switched onto lines in Glasgow which are already among Scotland’s busiest to get to low-level platforms at Queen Street for more than four months from Sunday, 20 March.
Other trains will be diverted to Glasgow Central.
Mr Verster told Holyrood’s infrastructure committee: “The timetable is going to be very tight. With the way I am cycling rolling stock and services through areas from Bellgrove to Hyndland, if I have a train failure there, the plan is dead.”
The tunnel work is to replace worn-out track.