My guess would be portable ventilation equipment - already commonly used for maintenance access to underground utility infrastructure. They will probably need t

My guess would be portable ventilation equipment - already commonly used for maintenance access to underground utility infrastructure. They will probably need to provide access to the tunnels from above at reasonably frequent intervals for maintenance access (e.g. to "unstick" disabled vehicles) but since these wouldn't be used for evacuation of more than a small number of people requirements would be very relaxed. The same accesses would serve to run blower ducts when work is being performed, and workers would wear gas monitors. This is all pretty standard for telecoms tunnels, sewers, etc. Safety evacuation requirements are also easier to meet when you limit tunnel access to a trained workforce under a permit system, compared to a situation where you have the general public in the tunnels. Emergency rebreathers, for example, can become part of your plan. A higher level of safety can be assured for workers since a permit system means that a control center will have positive accountability of everyone in harm's way.