Doesn’t even have to be solely trains, but a few lanes of busses would help with shorter/End of Line trips as well.
Though this is worth noting that ventilation concerns with tunnels are always a nightmare (one of the reasons there aren’t tons of tunnels). Its now turning into an issue where roads cross our 400 series highways, and one of the reasons they’re often bridges. As the highway reaches ~4lanes each way, they need to install ventilation of some kind, which has caused problems and forced design adjustments. Look at Barrie where they have a few roads running under the 400. Now that they’re trying to widen to 8 lanes, all those crossings are extending into “tunnel” category and have additional requirements. They’ve been forced to split the highway with an open air section in the middle over those roads in order to ensure proper airflow. This means more land acquisition and higher pricing to build.
I could be off, but I seem to remember that the requirements for ventilation are less if there is no combustion engine + exhaust fumes present. I’ll see if if I can dig up the actual design standards.